[mips] Fix lowering a signed immediate for *.d MSA instructions

The `lowerMSASplatImm` function zero-extends `i32` immediates while
building constant. If target type is `i64`, negative immediate loses
the sign. As a result, for example `__builtin_msa_ldi_d(-1)` lowered
to series of instruction loads incorrect value 0xffffffff to the `$w0`
register instead of single `ldi.d $w0, -1` instruction.

The fix zero-extends unsigned immediates and signed-extend signed
immediates.

Differential Revision: http://reviews.llvm.org/D59884

llvm-svn: 357264