Delay predication of stores until near the end of vector code generation

Delay predication of stores until near the end of vector code generation

Predicating stores requires creating extra blocks. It's much cleaner if we do this in one pass instead of mutating the CFG while writing vector instructions.

Besides which we can make use of helper functions to update domtree for us, reducing the work we need to do.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247139 91177308-0d34-0410-b5e6-96231b3b80d8