[Sparc] Custom bitcast between f64 and v2i32

Summary:
Currently bitcasting constants from f64 to v2i32 is done by storing the
value to the stack and then loading it again. This is not necessary, but
seems to happen because v2i32 is a valid type for Sparc V8. If it had not
been legal, we would have gotten help from the type legalizer.

This patch tries to do the same work as the legalizer would have done by
bitcasting the floating point constant and splitting the value up into a
vector of two i32 values.

Reviewers: venkatra, jyknight

Reviewed By: jyknight

Subscribers: glaubitz, fedor.sergeev, jrtc27, llvm-commits

Differential Revision: https://reviews.llvm.org/D49219

llvm-svn: 340723