[x86] reassociate integer multiplies using machine combiner pass

[x86] reassociate integer multiplies using machine combiner pass

Add i16, i32, i64 imul machine instructions to the list of reassociation
candidates.

A new bit of logic is needed to handle integer instructions: they have an
implicit EFLAGS operand, so we have to make sure it's dead in order to do
any reassociation with integer ops.

Differential Revision: http://reviews.llvm.org/D11660

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243756 91177308-0d34-0410-b5e6-96231b3b80d8