When moving 0.0 into a float vector, we can use to vi*gpr variants of
INS. I am not sure if we can easily express this in the tablegen
descriptions, because INS*vi*gpr is only defined for integer vectors and
I am not sure how to convert things there.
This patch extends LowerINSERT_VECTOR_ELT to bitcast the input float
vector to an integer vector, apply the insertion and bitcast the result
This way, we can piggy-back on the matching for the integer variants.