[llvm] [RISCV][GISel] Add calling convention support for half (PR #94110)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

Hi @dtcxzyw

If you compile with EXPENSIVE_CHECKS on and run the new testcase
 CodeGen/RISCV/GlobalISel/irtranslator/calling-conv-half.ll
it fails with
```
*** Bad machine code: Copy Instruction is illegal with mismatching sizes ***
- function:    caller_half_return_stack2
- basic block: %bb.1  (0x55852fd10918)
- instruction: $x10 = COPY %0:_(s16)
Def Size = 32, Src Size = 16
LLVM ERROR: Found 1 machine code errors.
```
Can also be seen if you just add ```-verify-machineinstrs``` to the last RUN line in the testcase.

https://github.com/llvm/llvm-project/pull/94110