Add [[maybe_unused]], remove unused check prefix

An unused check prefix apparently causes an error? Not sure if it was
always like that.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -2102,7 +2102,7 @@ void RISCVDAGToDAGISel::Select(SDNode *Node) {
       SubVecContainerVT = TLI.getContainerForFixedLengthVector(SubVecVT);
 #ifndef NDEBUG
       TypeSize VecRegSize = TypeSize::getScalable(RISCV::RVVBitsPerBlock);
-      bool ExactlyVecRegSized =
+      [[maybe_unused]] bool ExactlyVecRegSized =
           Subtarget->expandVScale(SubVecVT.getSizeInBits())
               .isKnownMultipleOf(Subtarget->expandVScale(VecRegSize));
       assert(isPowerOf2_64(Subtarget->expandVScale(SubVecVT.getSizeInBits())

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shuffle-concat.ll

@@ -1,9 +1,9 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
-; RUN: llc < %s -mtriple=riscv32 -mattr=+v -verify-machineinstrs | FileCheck -check-prefixes=CHECK,VLA %s
-; RUN: llc < %s -mtriple=riscv64 -mattr=+v -verify-machineinstrs | FileCheck -check-prefixes=CHECK,VLA %s
+; RUN: llc < %s -mtriple=riscv32 -mattr=+v -verify-machineinstrs | FileCheck -check-prefix=VLA %s
+; RUN: llc < %s -mtriple=riscv64 -mattr=+v -verify-machineinstrs | FileCheck -check-prefix=VLA %s
 
-; RUN: llc < %s -mtriple=riscv32 -mattr=+v -riscv-v-vector-bits-max=128 -verify-machineinstrs | FileCheck -check-prefixes=CHECK,VLS %s
-; RUN: llc < %s -mtriple=riscv64 -mattr=+v -riscv-v-vector-bits-max=128 -verify-machineinstrs | FileCheck -check-prefixes=CHECK,VLS %s
+; RUN: llc < %s -mtriple=riscv32 -mattr=+v -riscv-v-vector-bits-max=128 -verify-machineinstrs | FileCheck -check-prefix=VLS %s
+; RUN: llc < %s -mtriple=riscv64 -mattr=+v -riscv-v-vector-bits-max=128 -verify-machineinstrs | FileCheck -check-prefix=VLS %s
 
 define <8 x i32> @concat_2xv4i32(<4 x i32> %a, <4 x i32> %b) {
 ; VLA-LABEL: concat_2xv4i32: