As of 20b5728b7b1ccc4509a316efb270d46cc9526d69, C always enables Zca, so
the check `C || Zca` is equivalent to just checking for `Zca`.

This replaces any uses of `HasStdExtCOrZca` with a new `HasStdExtZca`
(with the same assembler description, to avoid changes in error
messages), and simplifies everywhere where C++ needed to check for
either C or Zca.

The Subtarget function is just deprecated for the moment.

Stack clash protection code was missing from
RISCVFrameLowering::eliminateCallFramePseudoInstr, calling allocateStack
fixes it.
This patch also fixes the tests in stack-probing-dynamic.ll that should
be testing the stack allocation before a function call.

The expensive checks bots found issues with #132481, due to not setting
defs/uses correctly. In 31bd7a507152 I added verify flags, so that the
failure is reproduced without requiring expensive checks, and xfailed
the test.

This change:
- Ensures that registers are correctly marked as defs/uses.
- Removes the xfail.
- Leaves the tests with `-verify-machineinstrs` which should have been
present originally.

The `qc.c.mienter` and `qc.c.mienter.nest` instructions, broadly only
save the argument and temporary registers. The exceptions are that they
also save `fp` (`s0`) to construct a frame chain from the signal handler
to the frame below, and they also save `ra`. They are designed this way
so that (if needed) push and pop instructions can be used to save the
callee-saved registers.

This patch implements this optimisation, constructing the following
rather than a long sequence of `sw` and `lw` instructions for saving the
callee-saved registers:

```asm
  qc.c.mienter
  qc.cm.push {ra, s0-sN}, -M
  ...
  qc.cm.pop {ra, s0-sN}, M
  qc.c.mileaveret
```

There are some carefully-worked-out details here, especially around CFI
information. For any register saved by both `qc.c.mienter(.nest)` and
the push (which is `ra` and `s0` at most), we point the CFI information
at the version saved by `qc.c.mienter(.nest)`. This ensures the CFI
points at the same `fp` copy as a frame pointer unwinder would find.

This Change adds support for two SiFive vendor attributes in clang:
- "SiFive-CLIC-preemptible"
- "SiFive-CLIC-stack-swap"

These can be given together, and can be combined with "machine", but
cannot be combined with any other interrupt attribute values.

These are handled primarily in RISCVFrameLowering:
- "SiFive-CLIC-stack-swap" entails swapping `sp` with `sf.mscratchcsw`
  at function entry and exit, which holds the trap stack pointer.
- "SiFive-CLIC-preemptible" entails saving `mcause` and `mepc` before
  re-enabling interrupts using `mstatus`. To save these, `s0` and `s1`
  are first spilled to the stack, and then the values are read into
  these registers. If these registers are used in the function, their
  values will be spilled a second time onto the stack with the generic
  callee-saved-register handling. At the end of the function interrupts
  are disabled again before `mepc` and `mcause` are restored.

This Change also adds support for the following two experimental
extensions, which only contain CSRs:
- XSfsclic - for SiFive's CLIC Supervisor-Mode CSRs
- XSfmclic - for SiFive's CLIC Machine-Mode CSRs

The latter is needed for interrupt support.

The CFI information for this implementation is not correct, but I'd
prefer to correct this in a follow-up. While it's unlikely anyone wants
to unwind through a handler, the CFI information is also used by
debuggers so it would be good to get it right.

Co-authored-by: Ana Pazos <apazos@quicinc.com>

This reverts commit 50cdf6cbc5035345507bb4d23fcb0292272754eb.

This patch causes miscompiles with vector and produces some odd code for
ilp32e.

Reuse existing local variable.

We can't assume MBBI is still pointing at MBB if we've already expanded
a probe. We need to re-query the MBB from MBBI. Fixes #135206

Co-authored-by: Craig Topper <craig.topper@sifive.com>