[PATCH] Fix DCE REG_LIBCALL note moving from noop move insns (PR rtl-optimization/33644)

[Richard Kenner]

> __tls_get_addr is not const, and not pure either.

Why not?  Doesn't it return the same value every time?  If it DOESN'T,
then how can we CSE it?  And if it DOES, why don't we mark it that way?