[committed] Fix reload assert due to CLOBBER_HIGH support

[Jeff Law]

forget_old_reloads_1 can be called with NULL_RTX for SETTER which causes
a segfault in the assert that SETTER is not a CLOBBER_HIGH expression
(see the call from within reload_as_needed)


 forget_old_reloads_1 (XEXP (in_reg, 0), NULL_RTX, NULL);


Clearly this only happens on reload targets.  Verified this fixed the
segfault my tester hit on c6x.

Installing as obvious.

JEff