[PowerPC] Replace float load/store pair with integer load/store pair when it's...

[PowerPC] Replace float load/store pair with integer load/store pair when it's only used in load/store

Replace float load/store pair with integer load/store pair when it's only used in load/store,
because float load/store instructions cost more cycles then integer load/store.

A typical scenario is when there is a call with more than 13 float arguments passing, we need pass them by stack.
So we need a load/store pair to do such memory operation if the variable is global variable.

Differential Revision: https://reviews.llvm.org/D64195

llvm-svn: 366775