[AMDGPU][MC] Corrected decoding of 16-bit literals
16-bit literals are encoded as 32-bit values. If high 16-bits of the value is 0xFFFF, the decoded instruction cannot be reassembled.
For example, the following code
0xff,0x04,0x04,0x52,0xcd,0xab,0xff,0xff
was decoded as
v_mul_lo_u16_e32 v2, 0xffffabcd, v2
However this literal is actually a 64-bit constant 0x00000000ffffabcd which violates requirements described in the documentation - the truncation is not safe.
This change corrects decoding to make reassembly possible.
Reviewers: arsenm, rampitec
Differential Revision: https://reviews.llvm.org/D84098