[AArch64] Guard against getRegisterBitWidth returning zero in vector instr cost. (#117749)

If the getRegisterBitWidth is zero (such as in sme streaming functions),
then we could hit a crash from using % RegWidth.

Author: davemgreen

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

@@ -3259,7 +3259,7 @@ InstructionCost AArch64TTIImpl::getVectorInstrCostHelper(
       auto RegWidth =
           getRegisterBitWidth(TargetTransformInfo::RGK_FixedWidthVector)
               .getFixedValue();
-      return (Idx == 0 || (Idx * EltSz) % RegWidth == 0);
+      return Idx == 0 || (RegWidth != 0 && (Idx * EltSz) % RegWidth == 0);
     };
 
     // Check if the type constraints on input vector type and result scalar type

llvm/test/Analysis/CostModel/AArch64/extract_float_streaming.ll

@@ -0,0 +1,18 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes="print<cost-model>" 2>&1 -disable-output -mtriple=aarch64-unknown-linux -mattr=+sme | FileCheck %s
+
+define double @extract_case7(<4 x double> %a) "aarch64_pstate_sm_enabled" {
+; CHECK-LABEL: 'extract_case7'
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %0 = extractelement <4 x double> %a, i32 1
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %1 = extractelement <4 x double> %a, i32 2
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %res = fmul double %0, %1
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: ret double %res
+;
+entry:
+  %1 = extractelement <4 x double> %a, i32 1
+  %2 = extractelement <4 x double> %a, i32 2
+  %res = fmul double %1, %2
+  ret double %res
+}
+
+declare void @foo(double)