Fix bug that prohibited assignments with pointer expressions on the LHS

Previously, code like "*(ptr+1) = foo" would claim that the LHS was invalid
for an assignment expression.

Issue #138.

tests/ptr-assign-lhs-math-1.ispc

@@ -0,0 +1,16 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform float a[programCount];
+    a[programIndex] = aFOO[programIndex];
+
+    uniform float * uniform ptr = a;
+    *(ptr+1) = 0;
+    RET[programIndex] = a[programIndex];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1+programIndex;
+    RET[1] = 0;
+}

tests/ptr-assign-lhs-math-2.ispc

@@ -0,0 +1,15 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform float a[programCount];
+    a[programIndex] = aFOO[programIndex];
+
+    uniform float * varying ptr = a;
+    *(ptr+programIndex) = 0;
+    RET[programIndex] = a[programIndex];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}