HLSL: Fix #942: Map SV_TargetN to SPV Location N.

Test/hlsl.target.frag

@@ -0,0 +1,10 @@
+struct PSInput {
+  float interp;
+  uint no_interp;
+};
+
+void main(PSInput input : INPUT, out float4 out1 : SV_TARGET1, out float4 out2 : SV_TARGET3)
+{
+    out1 = 1;
+    out2 = 0;
+}