compiler: rename Compiler.getValue -> builder.getValue

This is a fairly big commit, but it actually changes very little.
getValue should really be a property of the builder (or frame), where
the previously created instructions are kept.

compiler/interrupt.go

@@ -24,7 +24,7 @@ func (c *Compiler) emitInterruptGlobal(frame *Frame, instr *ssa.CallCommon) (llv
 	// Note that bound functions are allowed if the function has a pointer
 	// receiver and is a global. This is rather strict but still allows for
 	// idiomatic Go code.
-	funcValue := c.getValue(frame, instr.Args[1])
+	funcValue := frame.getValue(instr.Args[1])
 	if funcValue.IsAConstant().IsNil() {
 		// Try to determine the cause of the non-constantness for a nice error
 		// message.