[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