kill dead stores when coalescing slots

This is necessary because, post
fusion, dead stores may clobber
data.  A new test case exposes
one such situation.

test/mem2.ssa

@@ -0,0 +1,32 @@
+# Ember Sawady reported this bug
+# in stack-slot coalescing
+
+type :t = { w 2 }
+
+function :t $func() {
+@start.0
+	%temp =l alloc4 4
+	%ret =l alloc4 8
+	storew 1, %temp
+	# storew can also go here
+	%field =l add %ret, 4
+	storew 2, %ret
+	blit %temp, %field, 4
+	# removing either of these storews causes it to work
+	storew 2, %ret
+	ret %ret
+}
+
+export function w $main() {
+@start
+	%ret =:t call $func()
+	%fptr =l add %ret, 4
+	%field =w loaduw %fptr
+	%x =w ceqw %field, 1
+	jnz %x, @passed, @failed
+@failed
+	# this fails despite 1 => temp => ret + 4 => field
+	call $abort()
+@passed
+	ret 0
+}