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

[Richard Kenner]

> Things like a multiplication can be CSE-ed in tree-ssa, so there
> should not be any libcalls of this kind to CSE in the generated RTL.
> 
> But TLS addressing is not exposed in GIMPLE so you can't CSE the TLS
> addresses either.

That's true, but we aren't depending *just* on cse'ing stuff at the tree
level, are we?  We do cse multiplication libcalls at the RTL level and that
same approach should work for TLS libcalls.

> Compile with -fpic.  There should only be one call to __tls_get_addr
> to get a base address.  Without libcalls there will be two calls.  The
> base address is not exposed in tree-ssa.

But isn't the function marked const?  (Is that the right flag?)