Carve out a place in instcombine to put transformations which work knowing that their

Carve out a place in instcombine to put transformations which work knowing that their
result is non-zero.  Implement an example optimization (PR9814), which allows us to
transform:
  A / ((1 << B) >>u 2)
into:
  A >>u (B-2)

which we compile into:

_divu3:                                 ## @divu3
	leal	-2(%rsi), %ecx
	shrl	%cl, %edi
	movl	%edi, %eax
	ret

instead of:

_divu3:                                 ## @divu3
	movb	%sil, %cl
	movl	$1, %esi
	shll	%cl, %esi
	shrl	$2, %esi
	movl	%edi, %eax
	xorl	%edx, %edx
	divl	%esi, %eax
	ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131860 91177308-0d34-0410-b5e6-96231b3b80d8