- _Float16 is now accepted by Clang.

- The half IR type is fully handled by the backend.

- These values are passed in FP registers and converted to/from float around
  each operation.

- Compiler-rt conversion functions are now built for s390x including the missing
  extendhfdf2 which was added.

Fixes #50374

As part of an effort to enable instr-ref-based debug value tracking,
this PR implements `SystemZInstrInfo::isLoadFromStackSlotPostFE`, as
well as `SystemZInstrInfo::isStoreToStackSlotPostFE`. The implementation
relies upon the presence of MachineMemoryOperands on the relevant
`MachineInstr`s in order to access the `FrameIndex` post frame index
elimination.

Since these new functions are only meant to be called after frame-index
elimination, they assert against the present of a frame index on the
base register operand of the instruction.

Outside of the utility of these functions to enable instr-ref-based
debug value tracking, they also changes the behavior of the AsmPrinter,
since it will now be able to properly detect non-folded spills and
reloads, so this changes a number of tests that were checking
specifically for folded reloads.

Note that there are some tests that still check for `vst` and `vl` as
folded spills/reloads even though they should be straight reloads. This
will be addressed in a future PR.

Co-authored-by: Dominik Steenken <dominik.steenken@gmail.com>

The lit test fmuladd-soft-float.ll only specifies s390x as platform,
but the test is Linux specific, causing problems when run on z/OS.
This change updates the triple to fix this.

The previous behavior could be harmful in some edge cases, such as
emitting a call to `fma()` in the `fma()` implementation itself.

Do this by just being more accurate in `isFMAFasterThanFMulAndFAdd()`.
This was already done for PowerPC; this commit just extends that to Arm,
z/Arch, and x86. MIPS and SPARC already got it right, but I added tests
for them too, for good measure.

Note: I don't have commit access.