[PowerPC] Unify handling for conversion of FP_TO_INT feeding a store

Existing DAG combine only handles conversions for FP_TO_SINT:
"{f32, f64} x { i32, i16 }"

This patch simplifies the code to handle:
"{ FP_TO_SINT, FP_TO_UINT } x { f64, f32 } x { i64, i32, i16, i8 }"

Differential Revision: https://reviews.llvm.org/D46102

llvm-svn: 331778