match.pd: Fold pattern of round semantics.

match.pd: Fold pattern of round semantics.

In the 538.imagick_r benchmark of Spec2017, I find these pattern from
MagickRound function. This patch implements these pattern in match.pd
for 4 rules:
1) (x-floor(x)) < (ceil(x)-x) ? floor(x) : ceil(x) -> floor(x+0.5)
2) (x-floor(x)) >= (ceil(x)-x) ? ceil(x) : floor(x) -> floor(x+0.5)
3) (ceil(x)-x) > (x-floor(x)) ? floor(x) : ceil(x) -> floor(x+0.5)
4) (ceil(x)-x) <= (x-floor(x)) ? ceil(x) : floor(x) -> floor(x+0.5)

The patch implements floor(x+0.5) operation to replace these pattern
that semantics of round(x) function.

The patch was regtested on aarch64-linux-gnu and x86_64-linux-gnu,
SPEC 2017 and SPEC 2006 were run:
As for SPEC 2017, 538.imagick_r benchmark performance increased by 3%+
in base test of ratio mode.
As for SPEC 2006, while the transform does not seem to be triggered,
we also see no non-noise impact on performance.

gcc/ChangeLog:

	* match.pd: Add new pattern for round.

gcc/testsuite/ChangeLog:

	* gcc.dg/fold-round-1.c: New test.