xtensa: Make one_cmplsi2 optimizer-friendly

xtensa: Make one_cmplsi2 optimizer-friendly

In Xtensa ISA, there is no single machine instruction that calculates unary
bitwise negation.  But a few optimizers assume that bitwise negation can be
done by a single insn.

As a result, '((x < 0) ? ~x : x)' cannot be optimized to '(x ^ (x >> 31))'
ever before, for example.

This patch relaxes such limitation, by putting the insn expansion off till
the split pass.

gcc/ChangeLog:

	* config/xtensa/xtensa.md (one_cmplsi2):
	Rearrange as an insn_and_split pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/xtensa/one_cmpl_abs.c: New.