[MIPS] Use custom bitcast lowering to avoid excessive instructions

On Mips32r2 bitcast can be expanded to two sw instructions and an ldc1
when using bitcast i64 to double or an sdc1 and two lw instructions when
using bitcast double to i64. By introducing custom lowering that uses
mtc1/mthc1 we can avoid excessive instructions.

Patch by Mirko Brkusanin.

Differential Revision: https://reviews.llvm.org/D61069

llvm-svn: 359171