[VP] Fix VPintrinsic::getStaticVectorLength for vp.merge|select

VPIntrinsic::getStaticVectorLength infers the operational vector length
of a VPIntrinsic instance from a type that is used with the intrinsic.
The function used the mask operand before. Yet, vp.merge|select do not
have a mask operand (in the predicating sense that the other VP
intrinsics are using them - it is a selection mask for them). Fallback
to the return type to fix this.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D121913

llvm/include/llvm/IR/VPIntrinsics.def

@@ -397,6 +397,11 @@ HELPER_REGISTER_REDUCTION_SEQ_VP(vp_reduce_fmul, VP_REDUCE_FMUL,
 
 ///// Shuffles {
 
+// The mask 'cond' operand of llvm.vp.select and llvm.vp.merge are not reported
+// as masks with the BEGIN_REGISTER_VP_* macros.  This is because, unlike other
+// VP intrinsics, these two have a defined result on lanes where the mask is
+// false.
+//
 // llvm.vp.select(cond,on_true,on_false,vlen)
 BEGIN_REGISTER_VP(vp_select, None, 3, VP_SELECT, -1)
 VP_PROPERTY_FUNCTIONAL_OPC(Select)

llvm/lib/IR/IntrinsicInst.cpp

@@ -299,7 +299,12 @@ ElementCount VPIntrinsic::getStaticVectorLength() const {
   };
 
   Value *VPMask = getMaskParam();
-  assert(VPMask && "No mask param?");
+  if (!VPMask) {
+    assert((getIntrinsicID() == Intrinsic::vp_merge ||
+            getIntrinsicID() == Intrinsic::vp_select) &&
+           "Unexpected VP intrinsic without mask operand");
+    return GetVectorLengthOfType(getType());
+  }
   return GetVectorLengthOfType(VPMask->getType());
 }