[RISCV] Change function alignment to 4 bytes, and 2 bytes for RVC

[kito-cheng]

Summary

According RISC-V ELF psABI specification, base RV32 and RV64 ISAs only allow 32-bit instruction alignment, but allow to be aligned to 16-bit boundaries for C-extension.

So we just align to 4 bytes and 2 bytes for C-extension is enough.

[kito-cheng]

Changes:

• Add test case.

[asb]

Thanks Kito, looks good to me.

lib/Target/RISCV/RISCVISelLowering.cpp

RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
 
   setOperationAction(ISD::GlobalAddress, XLenVT, Custom);
   setOperationAction(ISD::BlockAddress, XLenVT, Custom);
   setOperationAction(ISD::ConstantPool, XLenVT, Custom);
 
   setBooleanContents(ZeroOrOneBooleanContent);
 
   // Function alignments (log2).
-  setMinFunctionAlignment(3);
-  setPrefFunctionAlignment(3);
+  unsigned FunctionAlignment = Subtarget.hasStdExtC() ? 1 : 2;
+  setMinFunctionAlignment(FunctionAlignment);
+  setPrefFunctionAlignment(FunctionAlignment);
 
   // Effectively disable jump table generation.
   setMinimumJumpTableEntries(INT_MAX);
 }
 
 EVT RISCVTargetLowering::getSetCCResultType(const DataLayout &DL, LLVMContext &,
                                             EVT VT) const {
   if (!VT.isVector())