optimize @memset with undefined in safety-checked modes

[andrewrk]

I believe this is one of the causes of GeneralPurposeDebugAllocator's slowness:

zig/std/mem.zig

Lines 107 to 110 in 9dfd24a

	// This loop gets optimized out in ReleaseFast mode
	for (byte_slice) |*byte| {
	byte.* = undefined;
	}

By default this has Valgrind integration, so this loop is doing a lot of client requests. Instead it should be able to do @memset(byte_slice.ptr, undefined, byte_slice.len) and this would have the same effect but only do 1 Valgrind client request.