[Alignment] fix dubious min function alignment

[gchatelet]

Summary

This was discovered while introducing the llvm::Align type.
The original setMinFunctionAlignment used to take alignment as log2, looking at the comment it seems like instructions are to be 2-bytes aligned and not 4-bytes aligned.

[uweigand]

LGTM.

[gchatelet]

Thx!

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

SystemZTargetLowering::SystemZTargetLowering(const TargetMachine &TM,
   // such as SystemZ has with CC, so set this to the register-pressure
   // scheduler, because it can.
   setSchedulingPreference(Sched::RegPressure);
 
   setBooleanContents(ZeroOrOneBooleanContent);
   setBooleanVectorContents(ZeroOrNegativeOneBooleanContent);
 
   // Instructions are strings of 2-byte aligned 2-byte values.
-  setMinFunctionAlignment(llvm::Align(4));
+  setMinFunctionAlignment(llvm::Align(2));
   // For performance reasons we prefer 16-byte alignment.
   setPrefFunctionLogAlignment(4);
 
   // Handle operations that are handled in a similar way for all types.
   for (unsigned I = MVT::FIRST_INTEGER_VALUETYPE;
        I <= MVT::LAST_FP_VALUETYPE;
        ++I) {
     MVT VT = MVT::SimpleValueType(I);