InstCombine: Combine mul with div.

We can combne a mul with a div if one of the operands is a multiple of
the other:

%mul = mul nsw nuw %a, C1
%ret = udiv %mul, C2
  =>
%ret = mul nsw %a, (C1 / C2)

This can expose further optimization opportunities if we end up
multiplying or dividing by a power of 2.

Consider this small example:

define i32 @f(i32 %a) {
  %mul = mul nuw i32 %a, 14
  %div = udiv exact i32 %mul, 7
  ret i32 %div
}

which gets CodeGen'd to:

    imull       $14, %edi, %eax
    imulq       $613566757, %rax, %rcx
    shrq        $32, %rcx
    subl        %ecx, %eax
    shrl        %eax
    addl        %ecx, %eax
    shrl        $2, %eax
    retq

We can now transform this into:
define i32 @f(i32 %a) {
  %shl = shl nuw i32 %a, 1
  ret i32 %shl
}

which gets CodeGen'd to:

    leal        (%rdi,%rdi), %eax
    retq

This fixes PR20681.