[CVP] Simplify umulo and smulo that cannot overflow

If a umul.with.overflow or smul.with.overflow operation cannot
overflow, simplify it to a simple mul nuw / mul nsw. After the
refactoring in D60668 this is just a matter of removing an
explicit check against multiplications.

Differential Revision: https://reviews.llvm.org/D60791

llvm-svn: 358521