[CodeGen] [AMDGPU] Adds pre-commit test for fmul-select combine

This adds the f32/f64/f16/bf16 test cases for below pattern :

fmul x, select(y, A, B)

where A & B could be inline/non-inline values

It acts as pre-commit tests for dagCombine above pattern into cheaper
ldexp in f64 case.

Author: vg0204