[PR117946][LRA]: When assigning hard reg use biggest mode to check ira_prohibited_cla...

[PR117946][LRA]: When assigning hard reg use biggest mode to check ira_prohibited_class_mode_regs

A pseudo in the PR test case gets hard reg 43 which is x86 r15 (after
r15, xmm regs go).  The pseudo is of INT_SSE_CLASS and SImode but is
used in TImode as paradoxical subreg.  r15 in TImode is wrong and does
not satisfy constraint 'r'.  Therefore LRA creates moves involving the
pseudo in TImode until the limit of reload insns is achieved.
Unfortunately x86 hard_regno_mode_ok (as some hooks for other targets)
says that it is ok to use r15 for TImode pseudo.  Therefore LRA uses
ira_prohibited_class_mode_regs for such cases but it was checked
against native pseudo mode.  The patch fixes it by using the biggest
pseudo mode.

gcc/ChangeLog:

	PR rtl-optimization/117946
	* lra-assigns.cc: (find_hard_regno_for_1): Use the biggest mode to
	check ira_prohibited_class_mode_regs.

gcc/testsuite/ChangeLog:

	PR rtl-optimization/117946
	* gcc.target/i386/pr117946.c: New.