SPV: RelaxedPrecision: Generalize fix #2293 to cover more operations.

This simplifies and enforces use of precision in many more places,
to help avoid accidental loss of RelaxedPrecision through intermediate
operations. Known fixes are:
- ?:
- function return values with mis-matched precision
- precision of function return values when a copy was needed to fix types

Test/spv.precisionArgs.frag

@@ -6,6 +6,11 @@ void fooConst(const in float f, const in highp float g) { }
 
 void foo(in float f, in highp float g) { }
 
+      float retM (      float x) { return x; }
+highp float retH (highp float x) { return x; }
+      float retHM(highp float x) { return x; }
+highp float retMH(      float x) { return x; }
+
 void main()
 {
     float aM, bM;
@@ -14,4 +19,9 @@ void main()
     fooConst(aH, bH);   // must copy aH
     foo(aM, bM);
     foo(aH, bH);
+
+    retM(aM);
+    retH(aH);
+    retHM(aH);
+    retMH(aM);
 }