[GlobalISel] Support for optimized pow is missing

**Godbolt** - https://godbolt.org/z/ez6nnoWcd
**Testcase**
```
define float @pow(float %x) nounwind {
%r = call nsz ninf afn float @llvm.pow.f32(float %x, float 2.5e-01)
ret float %r
}
```
**SDAG**
```
pow: # @pow
sqrtss %xmm0, %xmm0
sqrtss %xmm0, %xmm0
retq
```
Currently **GISEL** only have support for basic **libcall mapping**. Above case it is crashing due to missing legalizer support in **GISEL**.

Start with the GlobalISel legalizer handling for llvm.pow.f32 and compare it with the SDAG result shown in the issue. Use the linked Godbolt testcase to reproduce the crash, then add coverage for the x^0.25 case and verify that GlobalISel lowers it successfully with equivalent behavior.

Written by the indexing model from the issue text.