[LV] Fix missing entry in willGenerateVectors

willGenerateVectors switches on opcodes of a recipe, but Histogram is
missing in the switch statement, which could cause a crash in some
cases. The crash was initially observed when developing another patch:
no test case is included with this patch, as the bug is evident.

Author: artagnon

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4460,6 +4460,7 @@ static bool willGenerateVectors(VPlan &Plan, ElementCount VF,
       case VPDef::VPWidenSelectSC:
       case VPDef::VPBlendSC:
       case VPDef::VPFirstOrderRecurrencePHISC:
+      case VPDef::VPHistogramSC:
       case VPDef::VPWidenPHISC:
       case VPDef::VPWidenIntOrFpInductionSC:
       case VPDef::VPWidenPointerInductionSC: