Both x86 and AArch64 support converting from one sized GPR to another
sized FPR. Expose it in our IR op.
Fixes a bug in CVTSI2SS with a 64bit source and it was treating the
input source as a 32bit value.
eg:
0x0000'0000'FFFF'FFFF was treated as converting to -1 float, which was wrong since it is a 64bit value