[GlobalISel][X86] G_FCONSTANT support.

Summary: G_FCONSTANT support, port the implementation from X86FastIsel.

Reviewers: zvi, delena, guyblank

Reviewed By: delena

Subscribers: rovka, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D37734

llvm-svn: 313478