Previously, we'd bitcast e.g. a vector of floats to a vector of i32s and then
use the i32 variant of masked_load/masked_store/gather/scatter. Now, we have
separate float/double variants of each of those.
(i.e., stop just reusing the ones for AVX1).
For now the only difference is that the int/uint min/max
functions call the new intrinsic for that. Once gather is
available from LLVM, that will go here as well.
