[AMDGPU] Adds pre-commit test for fmul-select combine (llvm#111107)

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

`fmul x, select(y, A, B)`
with just one use of select Inst above.

It acts as pre-commit tests for dagCombining above pattern into cheaper
ldexp in case of non-inlline 32 bit-constants. (llvm#111109)

Author: vg0204