xtensa: Fix B[GE/LT]UI instructions with immediate values of 32768 or 65536 not being...

xtensa: Fix B[GE/LT]UI instructions with immediate values of 32768 or 65536 not being emitted

This is because in canonicalize_comparison() in gcc/expmed.cc, the COMPARE
rtx_cost() for the immediate values in the title does not change between
the old and new versions.  This patch fixes that.

(note: Currently, this patch only works if some constant propagation
optimizations are enabled (-O2 or higher) or if bare large constant
assignments are possible (-mconst16 or -mauto-litpools).  In the future
I hope to make it work at -O1...)

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_b4const_or_zero):
	Remove.
	(xtensa_b4const): Add a case where the value is 0, and rename
	to xtensa_b4const_or_zero.
	(xtensa_rtx_costs): Fix to also consider the result of
	xtensa_b4constu().

gcc/testsuite/ChangeLog:

	* gcc.target/xtensa/BGEUI-BLTUI-32k-64k.c: New.