volatile loads must not be simplified

volatile loads must not be simplified

memops.c:simplify_loads() tries to simplify all loads,
even volatile ones.
For example, on the following code:
	static int foo(volatile int *a, int v)
	{
		*a = v;
		return *a;
	}

test-linearize returns something like:
	foo:
		store.32    %arg2 -> 0[%arg1]
		ret.32      %arg2

while the correct output is more like:
	foo:
		store.32    %arg2 -> 0[%arg1]
		load.32     %r5 <- 0[%arg1]
		ret.32      %r5

The fix is to simply ignore loads with the 'volatile' modifier.

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