[InstCombine] Support (mul (sext x), cst) --> (sext (mul x, cst')) and (mul (zext x), cst) --> (zext (mul x, cst')) for vectors constants.

Similar to D51236, but for mul instead of add.

Differential Revision: https://reviews.llvm.org/D51900

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341961 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed