[PowerPC] Exploit store instructions that store a single vector element

This patch exploits the instructions that store a single element from a vector
to preform a (store (extract_elt)). We already have code that does this with
ISA 3.0 instructions that were added to handle i8/i16 types. However, we had
never exploited the existing ones that handle f32/f64/i32/i64 types.

Differential revision: https://reviews.llvm.org/D56175

llvm-svn: 352131