LoongArch: Define hook TARGET_COMPUTE_PRESSURE_CLASSES[PR120476].

LoongArch: Define hook TARGET_COMPUTE_PRESSURE_CLASSES[PR120476].

The rtx cost value defined by the target backend affects the
calculation of register pressure classes in the IRA, thus affecting
scheduling.  This may cause program performance degradation.
For example, OpenSSL 3.5.1 SHA512 and SPEC CPU 2017 exchange_r.

This problem can be avoided by defining a set of register pressure
classes in the target backend instead of using the default IRA to
automatically calculate them.

gcc/ChangeLog:

	PR target/120476
	* config/loongarch/loongarch.cc
	(loongarch_compute_pressure_classes): New function.
	(TARGET_COMPUTE_PRESSURE_CLASSES): Define.