[ScalarizeMaskedMemIntrin] Use cast instead of dyn_cast checked by an assert. Consistently make use of the element type variable we already have. NFCI

cast will take care of asserting internally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343277 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed