[MachineCombiner][RISCV] Relax optimization level requirement

Enable Machine Combiner for O1/O2/O3 optimization levels. It makes RISCV
consistent with other targets running Machine Combiner.

Originally it was enabled only for -O3, however I looked through time reports
and usually it takes 0.1%-0.4% of total time, and never takes more than 1.0%.

Differential Revision: https://reviews.llvm.org/D136339

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -268,7 +268,7 @@ void RISCVPassConfig::addPreEmitPass2() {
 
 void RISCVPassConfig::addMachineSSAOptimization() {
   TargetPassConfig::addMachineSSAOptimization();
-  if (TM->getOptLevel() == CodeGenOpt::Aggressive && EnableMachineCombiner)
+  if (EnableMachineCombiner)
     addPass(&MachineCombinerID);
 
   if (TM->getTargetTriple().getArch() == Triple::riscv64)

llvm/test/CodeGen/RISCV/machine-combiner-mir.ll

@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
 ; RUN: llc -mtriple=riscv64 -mattr=+d -verify-machineinstrs -mcpu=sifive-u74 \
-; RUN: -O3 -riscv-enable-machine-combiner=true        \
+; RUN: -O1 -riscv-enable-machine-combiner=true        \
 ; RUN: -stop-after machine-combiner < %s | FileCheck %s
 
 define double @test_reassoc_fadd1(double %a0, double %a1, double %a2, double %a3) {

llvm/test/CodeGen/RISCV/machine-combiner.ll

@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -mtriple=riscv64 -mattr=+d -verify-machineinstrs -mcpu=sifive-u74 \
-; RUN: -O3 -riscv-enable-machine-combiner=true < %s | \
+; RUN: -O1 -riscv-enable-machine-combiner=true < %s | \
 ; RUN: FileCheck %s
 
 define double @test_reassoc_fadd1(double %a0, double %a1, double %a2, double %a3) {