[PR104637] LRA: Split hard regs as many as possible on one subpass

[PR104637] LRA: Split hard regs as many as possible on one subpass

LRA hard reg split subpass is a small subpass used as the last
resort for LRA when it can not assign a hard reg to a reload
pseudo by other ways (e.g. by spilling non-reload pseudos).  For
simplicity the subpass works on one split base (as each split
changes pseudo live range info).  In this case it results in
reaching maximal possible number of subpasses.  The patch
implements as many non-overlapping hard reg splits
splits as possible on each subpass.

gcc/ChangeLog:

	PR rtl-optimization/104637
	* lra-assigns.cc (lra_split_hard_reg_for): Split hard regs as many
	as possible on one subpass.

gcc/testsuite/ChangeLog:

	PR rtl-optimization/104637
	* gcc.target/i386/pr104637.c: New.