GCN: Enable effective-target 'vect_early_break', 'vect_early_break_hw'

GCN: Enable effective-target 'vect_early_break', 'vect_early_break_hw'

Via XPASSing test cases after commit a657c7e3518fcfc796f223d47385cad5e97dc9a5
"testsuite: un-xfail TSVC loops that check for exit control flow vectorization":

    PASS: gcc.dg/vect/tsvc/vect-tsvc-s332.c (test for excess errors)
    PASS: gcc.dg/vect/tsvc/vect-tsvc-s332.c execution test
    [-XFAIL:-]{+XPASS:+} gcc.dg/vect/tsvc/vect-tsvc-s332.c scan-tree-dump vect "vectorized 1 loops"

    PASS: gcc.dg/vect/tsvc/vect-tsvc-s481.c (test for excess errors)
    PASS: gcc.dg/vect/tsvc/vect-tsvc-s481.c execution test
    [-XFAIL:-]{+XPASS:+} gcc.dg/vect/tsvc/vect-tsvc-s481.c scan-tree-dump vect "vectorized 1 loops"

    PASS: gcc.dg/vect/tsvc/vect-tsvc-s482.c (test for excess errors)
    PASS: gcc.dg/vect/tsvc/vect-tsvc-s482.c execution test
    [-XFAIL:-]{+XPASS:+} gcc.dg/vect/tsvc/vect-tsvc-s482.c scan-tree-dump vect "vectorized 1 loops"

..., it became apparent that GCN, too, does support vectorization of loops with
early breaks.  The relevant test cases are all-PASS with just the following
exceptions, to be looked into individually, later on:

    PASS: gcc.dg/vect/vect-early-break_25.c (test for excess errors)
    PASS: gcc.dg/vect/vect-early-break_25.c scan-tree-dump-times vect "vectorized 1 loops" 1
    FAIL: gcc.dg/vect/vect-early-break_25.c scan-tree-dump-times vect "Alignment of access forced using peeling" 1

    PASS: gcc.dg/vect/vect-early-break_56.c (test for excess errors)
    PASS: gcc.dg/vect/vect-early-break_56.c execution test
    XPASS: gcc.dg/vect/vect-early-break_56.c scan-tree-dump-times vect "vectorized 2 loops" 2

	gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_vect_early_break)
	(check_effective_target_vect_early_break_hw): Enable for GCN.