[X86] Use 64-bit version of source register in LowerPATCHABLE_EVENT_CALL and...

[X86] Use 64-bit version of source register in LowerPATCHABLE_EVENT_CALL and LowerPATCHABLE_TYPED_EVENT_CALL

Summary:
The PATCHABLE_EVENT_CALL uses i32 in the intrinsic. This
results in the register allocator picking a 32-bit register. We
need to use the 64-bit register when forming the MOV64rr
instructions. Otherwise we print illegal assembly in the text
output.

I think prior to this it was impossible for SrcReg to be equal
to DstReg so the NOP code was not reachable.

While there use Register instead of unsigned.

Also add a FIXME for what looks like a bug.

Reviewers: dberris

Reviewed By: dberris

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69365