Add a really quick hack at a machine code sinking pass, enabled with --enable-sinking.

Add a really quick hack at a machine code sinking pass, enabled with --enable-sinking.
It is missing validity checks, so it is known broken.  However, it is powerful enough
to compile this contrived code:

void test1(int C, double A, double B, double *P) {
  double Tmp = A*A+B*B;
  *P = C ? Tmp : A;
}

into:

_test1:
	movsd	8(%esp), %xmm0
	cmpl	$0, 4(%esp)
	je	LBB1_2	# entry
LBB1_1:	# entry
	movsd	16(%esp), %xmm1
	mulsd	%xmm1, %xmm1
	mulsd	%xmm0, %xmm0
	addsd	%xmm1, %xmm0
LBB1_2:	# entry
	movl	24(%esp), %eax
	movsd	%xmm0, (%eax)
	ret

instead of:

_test1:
	movsd	16(%esp), %xmm0
	mulsd	%xmm0, %xmm0
	movsd	8(%esp), %xmm1
	movapd	%xmm1, %xmm2
	mulsd	%xmm2, %xmm2
	addsd	%xmm0, %xmm2
	cmpl	$0, 4(%esp)
	je	LBB1_2	# entry
LBB1_1:	# entry
	movapd	%xmm2, %xmm1
LBB1_2:	# entry
	movl	24(%esp), %eax
	movsd	%xmm1, (%eax)
	ret

woo.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45570 91177308-0d34-0410-b5e6-96231b3b80d8