[mips][msa] Fix generation of bm(n)zi and bins[lr]i instructions

We have two cases here, the first one being the following instruction
selection from the builtin function:
bm(n)zi builtin -> vselect node -> bins[lr]i machine instruction

In case of bm(n)zi having an immediate which has either its high or low bits
set, a bins[lr] instruction can be selected through the selectVSplatMask[LR]
function. The function counts the number of bits set, and that value is
being passed to the bins[lr]i instruction as its immediate, which in turn
copies immediate modulo the size of the element in bits plus 1 as per specs,
where we get the off-by-one-error.

The other case is:
bins[lr]i -> vselect node -> bsel.v

In this case, a bsel.v instruction gets selected with a mask having one bit
less set than required.

Patch by Stefan Maksimovic.

Differential Revision: https://reviews.llvm.org/D30579

llvm-svn: 299768