GLSL/SPV: Fix #1310: don't create unnecessary integer matrices.

Test/spv.vecMatConstruct.frag

@@ -0,0 +1,14 @@
+#version 450
+
+void main()
+{
+    mat4x3 m;
+
+    vec2 v2 = vec2(m);
+    vec3 v3 = vec3(m);
+    vec4 v4 = vec4(m);
+
+    ivec2 iv2 = ivec2(m);
+    ivec3 iv3 = ivec3(m);
+    ivec4 iv4 = ivec4(m);
+}