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

[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

llvm-svn: 341961