[RISCV] Add branch folding before branch relaxation

This is a follow-up patch to PR llvm#133256.

This patch adds the branch folding pass after the newly added late
optimization pass for riscv, which reduces code size in all SPEC
benchmarks (except libm).

The improvements are: 500.perlbench_r (-3.37%), 544.nab_r (-3.06%),
557.xz_r (-2.82%), 523.xalancbmk_r (-2.64%), 520.omnetpp_r (-2.34%),
531.deepsjeng_r (-2.27%), 502.gcc_r (-2.19%), 526.blender_r (-2.11%),
538.imagick_r (-2.03%), 505.mcf_r (-1.82%), 541.leela_r (-1.74%),
511.povray_r (-1.62%), 510.parest_r (-1.62%), 508.namd_r (-1.57%),
525.x264_r (-1.47%).

Geo mean is -2.07%.

Some caveats:
* On llvm#131728 I mentioned a 7% improvement on execution time of xz, but
  that's no longer the case. I went back and also tried to reproduce the
  result with the code from llvm#131728 and couldn't. Now the results from
  that PR and this one are the same: an overall code size reduction but
  no exec time improvements.
* The root cause of the large number is not yet clear for me. I'm still
  investigating it.

Author: mikhailramalho

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -568,6 +568,7 @@ void RISCVPassConfig::addPreEmitPass() {
     addPass(createMachineCopyPropagationPass(true));
   if (TM->getOptLevel() >= CodeGenOptLevel::Default)
     addPass(createRISCVLateBranchOptPass());
+  addPass(&BranchFolderPassID);
   addPass(&BranchRelaxationPassID);
   addPass(createRISCVMakeCompressibleOptPass());
 }