[RISCV] Fix a UBSAN failure for passing INT64_MIN to std::abs.

[craig.topper]

Summary

clang recently started checking for INT64_MIN being passed to 64-bit std::abs.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

if (auto SimpleVID = isSimpleVIDSequence(Op)) {
     int64_t StepNumerator = SimpleVID->StepNumerator;
     unsigned StepDenominator = SimpleVID->StepDenominator;
     int64_t Addend = SimpleVID->Addend;
 
     assert(StepNumerator != 0 && "Invalid step");
     bool Negate = false;
     int64_t SplatStepVal = StepNumerator;
     unsigned StepOpcode = ISD::MUL;
-    if (StepNumerator != 1) {
-      if (isPowerOf2_64(std::abs(StepNumerator))) {
-        Negate = StepNumerator < 0;
-        StepOpcode = ISD::SHL;
-        SplatStepVal = Log2_64(std::abs(StepNumerator));
-      }
-    }
+    // Exclude INT64_MIN to avoid passing it to std::abs. We won't optimize it
+    // anyway as the shift of 63 won't fit in uimm5.
+    if (StepNumerator != 1 && StepNumerator != INT64_MIN &&
+        isPowerOf2_64(std::abs(StepNumerator))) {
 
     // Only emit VIDs with suitably-small steps/addends. We use imm5 is a
     // threshold since it's the immediate value many RVV instructions accept.
     // There is no vmul.vi instruction so ensure multiply constant can fit in
     // a single addi instruction.
     if (((StepOpcode == ISD::MUL && isInt<12>(SplatStepVal)) ||
          (StepOpcode == ISD::SHL && isUInt<5>(SplatStepVal))) &&
         isPowerOf2_32(StepDenominator) &&