[mips][msa] Accept more values for constant splats

This patches teaches the MIPS backend to accept more values for constant
splats. Previously, only 10 bit signed immediates or values that could be
loaded using an ldi.[bhwd] instruction would be acceptted. This patch relaxes
that constraint so that any constant value that be splatted is accepted.

As a result, the constant pool is used less for vector operations, and the
suite of bit manipulation instructions b(clr|set|neg)i can now be used with
the full range of their immediate operand.

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D30640

llvm-svn: 297457