AMDGPU/MC: Add assembler tests for v2f32 and v2b32 with imm operand

Add test coverage for https://github.com/llvm/llvm-project/issues/60563.
D142636 introduced a bug: incorrect disassembly of floating point inline
constants for v_pk_mov_b32, v_pk_add_f32, v_pk_mul_f32 and v_pk_fma_f32.
Precommit for D143574.

Differential Revision: https://reviews.llvm.org/D143573