JIT: Allow helper calls that always throw to be marked as no-return (#100900)

Fixes #100458 by addressing two issues:

When flagging a call node as no-return with GTF_CALL_M_DOES_NOT_RETURN, we should always increment Compiler::optNoReturnCallCount to avoid asserts in Compiler::fgTailMergeThrows. Previously, we weren't doing this in a unified place, which seemed error-prone.
When incrementing the optNoReturnCallCount member of an inlinee compiler, ensure this information is propagated to the inlinee's parent compiler. In a similar vein, if we try to inline a call, and the inlinee compiler determines it does not return, make sure we increment optNoReturnCallCount in the parent compiler object if the inline fails -- we've kept the call, and we now know it doesn't return.
With these changes, I can now mark helper calls that always throw as no-return; this unblocks morph to convert BBJ_ALWAYS blocks with helper calls that throw into BBJ_THROW blocks, and has the nice side effect of improving the efficacy of throw merging. Since I was touching relevant code, I decided to improve our usage of GenTreeCall::IsHelperCall, too.

src/coreclr/jit/valuenum.cpp

@@ -12685,7 +12685,7 @@ bool Compiler::fgValueNumberSpecialIntrinsic(GenTreeCall* call)
 
 void Compiler::fgValueNumberCall(GenTreeCall* call)
 {
-    if (call->gtCallType == CT_HELPER)
+    if (call->IsHelperCall())
     {
         bool modHeap = fgValueNumberHelperCall(call);