[AArch64][AMDGPU][WebAssembly] Use static_cast instead of a reinterpret_cast...

[AArch64][AMDGPU][WebAssembly] Use static_cast instead of a reinterpret_cast to downcast in parseMachineFunctionInfo. NFC

static_cast is a little safer here since the compiler will
ensure we're casting to a class derived from
yaml::MachineFunctionInfo.

I believe this first appeared on AMDGPU and was copied to the
other two targets.

Spotted when it was being copied to RISCV in D123178.

Differential Revision: https://reviews.llvm.org/D123260