[PowerPC] Skip combining (uint_to_fp x) if x is not simple type

Current powerpc64le backend hits
```
Combining: t7: f64 = uint_to_fp t6
llc: llvm-project/llvm/include/llvm/CodeGen/ValueTypes.h:291:
llvm::MVT llvm::EVT::getSimpleVT() const: Assertion `isSimple() &&
"Expected a SimpleValueType!"' failed.
```
This patch fixes it by skipping combination if `t6` is not simple
type.
Fixed https://bugs.llvm.org/show_bug.cgi?id=47660.

Reviewed By: #powerpc, steven.zhang

Differential Revision: https://reviews.llvm.org/D88388