[MemCpyOpt] Fix handling of readnone byval arguments

If the call is readnone, then there may not be any MemoryAccess
associated with the call. Bail out in that case.

This fixes the issue reported at
https://reviews.llvm.org/D94376#2578312.

Author: nikic

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -1541,6 +1541,8 @@ bool MemCpyOptPass::processByValArgument(CallBase &CB, unsigned ArgNo) {
   MemCpyInst *MDep = nullptr;
   if (EnableMemorySSA) {
     MemoryUseOrDef *CallAccess = MSSA->getMemoryAccess(&CB);
+    if (!CallAccess)
+      return false;
     MemoryAccess *Clobber = MSSA->getWalker()->getClobberingMemoryAccess(
         CallAccess->getDefiningAccess(), Loc);
     if (auto *MD = dyn_cast<MemoryDef>(Clobber))

llvm/test/Transforms/MemCpyOpt/byval-readnone.ll

@@ -0,0 +1,18 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -memcpyopt -S -enable-memcpyopt-memoryssa=0 | FileCheck %s
+; RUN: opt < %s -memcpyopt -S -enable-memcpyopt-memoryssa=1 -verify-memoryssa | FileCheck %s
+
+%struct = type { i16 }
+
+declare i16 @g(%struct*) readnone
+
+define void @f() {
+; CHECK-LABEL: @f(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CALL:%.*]] = call i16 @g(%struct* byval(%struct) align 1 undef)
+; CHECK-NEXT:    ret void
+;
+entry:
+  %call = call i16 @g(%struct* byval(%struct) align 1 undef)
+  ret void
+}