For very large structs poisoning could end up generating instructions
requiring larger local var offsets than we can handle which would hit an
IMPL_LIMIT that throws InvalidProgramException. Switch to using rep
stosd (x86/x64)/memset helper (other platforms) when a local needs more
than a certain number of mov instructions to poison.

Also includes a register allocator change to mark killed registers as
modified in addRefsForPhysRegMask instead of by the (usually) calling
function, since this function is used directly in the change.

Fix dotnet#60852