Optimize splat of a scalar load into a shuffle of a vector load when it's legal. e.g.

vector_shuffle (scalar_to_vector (i32 load (ptr + 4))), undef, <0, 0, 0, 0>
=>
vector_shuffle (v4i32 load ptr), undef, <1, 1, 1, 1>

iff ptr is 16-byte aligned (or can be made into 16-byte aligned).

llvm-svn: 90984