gcse: Skip hardreg pre when the hardreg is never live [PR121095]HEADtrunkmaster

r15-6789-ge7f98d9603808b added a new RTL pass for hardreg PRE for the hard register
of FPM_REGNUM, this pass could get expensive if you have a large number of basic blocks
and the hard register was never live so it does nothing in the end.
In the aarch64 case, FPM_REGNUM is only used for FP8 related code so it has a high probability
of not being used. So skipping the pass for that register can improve both compile time and memory
usage.

Build and tested for aarch64-linux-gnu.

	PR middle-end/121095
gcc/ChangeLog:

	* gcse.cc (execute_hardreg_pre): Skip if the hardreg which is never live.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>

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