[mips][msa] Fix corner case for integer constant splats with undef values.

lowerBUILD_VECTOR() was treating integer constant splats as being legal
regardless of whether they had undef values. This caused instruction
selection failures when the undefs were legalized to zero, making the
constant non-splat.

Fixed this by requiring HasAnyUndef to be false for a integer constant
splat to be legal. If it is true, a new node is generated with the undefs
replaced with the necessary values to remain a splat.