[AMDGPU] Fix V_CMP_CLASS_F16_t16_e64 src1 type.

For V_CMP_CLASS_F16_t16_e64 and V_CMPX_CLASS_F16_t16_e64,
https://reviews.llvm.org/D133723 changed the value type of src1 from i32 to i16.
These src1 operands are 16 bits, therefore need to be encoded as true16
operands. So the _e32 type was correctly set to VGPR_32_Lo128.
In _e64 form the operand class went from
VSrc_b32 to VSrc_b16. For some reason, we cannot encode inline literals for
VSrc_b16, see 5f5f566b. In this phase of
the true16 implementation, VSrc_b16 and VSrc_b32 are still similar,
except from that quirk of inlines. So set the operand class to regain
that function.

Reviewed By: dp, arsenm

Differential Revision: https://reviews.llvm.org/D134897