xtensa: Add some dedicated patterns that correspond to GIMPLE canonicalizations

xtensa: Add some dedicated patterns that correspond to GIMPLE canonicalizations

This patch offers better RTL representations against straightforward
derivations from some tree optimizers' canonicalized forms.

- rounding up to even, such as '(x + (x & 1))', is canonicalized to
  '((x + 1) & -2)', but the former is one instruction less than the latter
  in Xtensa ISA.
- signed greater or equal to zero as logical value '((signed)x >= 0)',
  is canonicalized to '((unsigned)(x ^ -1) >> 31)', but the equivalent
  '(((signed)x >> 31) + 1)' is one instruction less.

gcc/ChangeLog:

	* config/xtensa/xtensa.md (*round_up_to_even):
	New insn-and-split pattern.
	(*signed_ge_zero): Ditto.