rs6000: Make CSE'ing __tls_get_addr calls possible

rs6000: Make CSE'ing __tls_get_addr calls possible

CSE does not consider calls, not even const calls.  This patch puts a
REG_EQUAL note on the pseudo we assign the __tls_get_addr result to,
so that those pseudos can be CSE'd and the extra calls deleted as dead
code.

CSE should really handle const calls directly, but it is stage 4.

	* config/rs6000/rs6000.c (rs6000_legitimize_tls_address): Add REG_EQUAL
	notes for the result of the __tls_get_addr calls.
	* config/rs6000/rs6000.md (unspec UNSPEC_TLS_GET_ADDR): New.

From-SVN: r269902