missing load simplification

missing load simplification

In memops:find_dominating_parents(), the 'load-load optimization'
(see commit cf07903a "Don't bother finding dominating loads if ...")
cause some loads simplification to be missed.
For example, with the following code:
	int foo(int *i, int *j)
	{
		*i = 6;
		*j = 1;

		do {
			if (*i != 6)
				(*i)++;
			(*i)++;
		} while (*i != *j);

		return *j;
	}

test-linearize returns something like:
	foo:
	.L0:
		<entry-point>
		store.32    $6 -> 0[%arg1]
		store.32    $1 -> 0[%arg2]
		br          .L1

	.L1:
		load.32     %r4 <- 0[%arg1]
		setne.32    %r5 <- %r4, $6
		br          %r5, .L4, .L5

	.L4:
		add.32      %r8 <- %r4, $1
		store.32    %r8 -> 0[%arg1]
		br          .L5

	.L5:
		load.32     %r10 <- 0[%arg1]
		add.32      %r11 <- %r10, $1
		store.32    %r11 -> 0[%arg1]
		load.32     %r15 <- 0[%arg2]
		setne.32    %r16 <- %r11, %r15
		br          %r16, .L1, .L6

	.L6:
		ret.32      %r15

where we can notice that the first load in .L5 is not needed,
the value could be retrieved from %r4 and %r8, like:
	@@ -8,15 +8,17 @@
	 .L1:
	 	load.32     %r4 <- 0[%arg1]
	 	setne.32    %r5 <- %r4, $6
	+	phisrc.32   %phi4 <- %r4
	 	br          %r5, .L4, .L5

	 .L4:
	 	add.32      %r8 <- %r4, $1
	 	store.32    %r8 -> 0[%arg1]
	+	phisrc.32   %phi5 <- %r8
	 	br          .L5

	 .L5:
	-	load.32     %r10 <- 0[%arg1]
	+	phi.32      %r10 <- %phi4, %phi5
	 	add.32      %r11 <- %r10, $1
	 	store.32    %r11 -> 0[%arg1]
	 	load.32     %r15 <- 0[%arg2]

The fix essentially consists in reverting commit cf07903a but on
memops.c's version of find_dominating_parents().

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>