[ScalarizeMaskedMemIntrin] Bitcast the mask to the scalar domain and use...

[ScalarizeMaskedMemIntrin] Bitcast the mask to the scalar domain and use scalar bit tests for the branches for expandload/compressstore.

Same as what was done for gather/scatter/load/store in r367489.
Expandload/compressstore were delayed due to lack of constant
masking handling that has since been fixed.

llvm-svn: 367738