Split the store of a wide value merged from an int-fp pair into multiple stores.

For the store of a wide value merged from a pair of values, especially int-fp pair,
sometimes it is more efficent to split it into separate narrow stores, which can
remove the bitwise instructions or sink them to colder places.

Now the feature is only enabled on x86 target, and only store of int-fp pair is
splitted. It is possible that the application scope gets extended with perf evidence
support in the future.

Differential Revision: https://reviews.llvm.org/D22840

llvm-svn: 280505