Fix invalid cast in instcombine UMul/ZExt idiom

Fixes https://bugs.llvm.org/show_bug.cgi?id=25454

Do not assume IRBuilder creates Instruction where it can create Value.
Do not assume idiom operands are constant, leave generalisation ot the IRBuilder.

Differential Revision: https://reviews.llvm.org/D35114


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307554 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed