Reverting because mis-compiles:
- https://github.com/llvm/llvm-project/pull/131837
- https://github.com/llvm/llvm-project/pull/146320
- https://github.com/llvm/llvm-project/pull/146337

The insertion point of COPY isn't always optimal and could eventually
lead to a worse block layout, see the regression test in the first
commit.

This change affects many architectures but the amount of total
instructions in the test cases seems too be slightly lower.

canHoistIVInc was made to only allow integer types to avoid a crash in
isIndexedLoadLegal/isIndexedStoreLegal due to them failing an assertion
in getValueType (or rather in MVT::getVT which gets called from that)
when passed a struct type. Adjusting these functions to pass
AllowUnknown=true to getValueType means we don't get an assertion
failure (MVT::Other is returned which TLI->isIndexedLoadLegal should
then return false for), meaning we can remove this check for integer
type.

Currently, given:
```cpp
uint64_t incb(uint64_t x) {
  return x+svcntb();
}
```
LLVM generates:
```gas
incb:
        addvl   x0, x0, #1
        ret
```
Which is equivalent to:
```gas
incb:
        incb    x0
        ret
```

However, on microarchitectures like the Neoverse V2 and Neoverse V3,
the second form (with INCB) can have significantly better latency and
throughput (according to their SWOG). On the Neoverse V2, for example,
ADDVL has a latency and throughput of 2, whereas some forms of INCB
have a latency of 1 and a throughput of 4. The same applies to DECB.
This patch adds patterns to prefer the cheaper INCB/DECB forms over
ADDVL where applicable.

Currently, given:
```cpp
svuint8_t foo(uint8_t *x) {
  return svld1(svptrue_b8(), x);
}
```
We generate:
```gas
foo:
  ptrue   p0.b
  ld1b    { z0.b }, p0/z, [x0]
  ret
```
However, on little-endian and with unaligned memory accesses allowed, we
could instead be using LDR as follows:
```gas
foo:
  ldr     z0, [x0]
  ret
```

The second form avoids the predicate dependency.
Likewise for other types and stores.

Pull Request: https://github.com/llvm/llvm-project/pull/121305

This PR removes tests with `br i1 undef` under
`llvm/tests/Transforms/Loop*, Lower*`.

Somewhat confusingly a `SCEVMulExpr` is a `SCEVNAryExpr`, so can have
> 2 operands. Previously, the vscale immediate matching did not check
the number of operands of the `SCEVMulExpr`, so would ignore any
operands after the first two.

This led to incorrect codegen (and results) for ArmSME in IREE
(https://github.com/iree-org/iree), which sometimes addresses things
that are a `vscale * vscale` multiple away. The test added with this
change shows an example reduced from IREE. The second write should
be offset from the first `16 * vscale * vscale` (* 4 bytes), however,
previously LSR dropped the second vscale and instead offset the write by
`#4, mul vl`, which is an offset of `16 * vscale` (* 4 bytes).

Precommit test for #100080.

Extends LoopStrengthReduce to recognize immediates multiplied by vscale, and query the current target for whether they are legal offsets for memory operations or adds.