Fixed issue with aliasing local variables

ISPC++ now produces valid code, or an appropriate error message, for all
of my test cases.

tests_ispcpp/Makefile

@@ -1,5 +1,5 @@
 CXX=g++
-CXXFLAGS=-std=c++11
+CXXFLAGS=-std=c++11 -O2
 
 ISPC=../ispc
 ISPCFLAGS=--target=sse4-x2 -O2 --arch=x86-64