Inefficient codegen in builder pattern with existed copyable base value

[CrazyboyQCD]

In a builder pattern code, when building with existed copyable base value, codegen isn't efficient like partial copy.
This can be reproduced on both argument value and constant value.

Full copy and patial copy:
C version: https://godbolt.org/z/z3fWsKhqE
Rust version: https://godbolt.org/z/MeGf8E5Gd

Existed value and non existed value:
C version: https://godbolt.org/z/9z3x3Kv5c
Rust version: https://godbolt.org/z/vWEaEfd3e

Real use case examaple in rust: https://godbolt.org/z/3q9jn5c1o

[dtcxzyw]

llvm-project/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

Lines 203 to 209 in 1469d82

	/// Returns true if the beginning of this instruction can be safely shortened
	/// in length.
	static bool isShortenableAtTheBeginning(Instruction *I) {
	// FIXME: Handle only memset for now. Supporting memcpy/memmove should be
	// easily done by offsetting the source address.
	return isa<AnyMemSetInst>(I);
	}

It can be fixed by supporting memcpy in tryToShortenBegin.