[mips] Sign extend i32 return values on MIPS64

[mips] Sign extend i32 return values on MIPS64

Override getTypeForExtReturn so that functions returning
an i32 typed value have it sign extended on MIPS64.

Also provide patterns to get rid of unneeded sign extensions
for arithmetic instructions which implicitly sign extend
their results.

Differential Revision: https://reviews.llvm.org/D48374

llvm-svn: 338019