[AMDGPU] Make use of composeSubRegIndices. NFCI. (#95548)

Simplify SIInstrInfo::buildExtractSubReg by building one COPY with a
composed subreg index instead of two COPYs.

Author: jayfoad

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -5651,24 +5651,9 @@ unsigned SIInstrInfo::buildExtractSubReg(
   DebugLoc DL = MI->getDebugLoc();
   Register SubReg = MRI.createVirtualRegister(SubRC);
 
-  if (SuperReg.getSubReg() == AMDGPU::NoSubRegister) {
-    BuildMI(*MBB, MI, DL, get(TargetOpcode::COPY), SubReg)
-      .addReg(SuperReg.getReg(), 0, SubIdx);
-    return SubReg;
-  }
-
-  // Just in case the super register is itself a sub-register, copy it to a new
-  // value so we don't need to worry about merging its subreg index with the
-  // SubIdx passed to this function. The register coalescer should be able to
-  // eliminate this extra copy.
-  Register NewSuperReg = MRI.createVirtualRegister(SuperRC);
-
-  BuildMI(*MBB, MI, DL, get(TargetOpcode::COPY), NewSuperReg)
-    .addReg(SuperReg.getReg(), 0, SuperReg.getSubReg());
-
+  unsigned NewSubIdx = RI.composeSubRegIndices(SuperReg.getSubReg(), SubIdx);
   BuildMI(*MBB, MI, DL, get(TargetOpcode::COPY), SubReg)
-    .addReg(NewSuperReg, 0, SubIdx);
-
+      .addReg(SuperReg.getReg(), 0, NewSubIdx);
   return SubReg;
 }