[PATCH][PR target/94542]Don't allow PC-relative addressing for TLS data

[acsawdey]

One of the things that address_to_insn_form() is used for is determining 
whether a PC-relative addressing instruction could be used. In 
particular predicate pcrel_external_address and function 
prefixed_paddi_p() both use it for this purpose. So what emerged in 
PR/94542 is that it should be looking to see if the associated 
symbol_ref is a TLS symbol of some kind. TLS symbols cannot be addressed 
with PC-relative. This patch fixes both places in address_to_insn_form() 
where it is looking at a symbol_ref.

Regression tests passed with trunk 
38e62001c576b8c6ba2e08eb4673d69ec4c5b0f9 on ppc64le power9, and 
PC-relative code is now correct. OK for trunk?

Thanks!
    Aaron

2020-04-10  Aaron Sawdey  <acsawdey@linux.ibm.com>

	PR target/94542
	* config/rs6000/rs6000.c (address_to_insn_form): Do not attempt to
	use PC-relative addressing for TLS references.
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