[PowerPC] Do not attempt to reuse load for 64-bit FP_TO_UINT without FPCVT

We call the function that attempts to reuse the conversion without checking
whether the target matches the constraints that the callee expects. This patch
adds the check prior to the call.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=43976

Differential revision: https://reviews.llvm.org/D77564

(cherry picked from commit 64b31d96)