vect: Make partial trapping ops use predication [PR96373]

vect: Make partial trapping ops use predication [PR96373]

PR96373 points out that a predicated SVE loop currently converts
trapping unconditional ops into unpredicated vector ops.  Doing
the operation on inactive lanes can then raise an exception.

As discussed in the PR trail, we aren't 100% consistent about
whether we preserve traps or not.  But the direction of travel
is clearly to improve that rather than live with it.  This patch
tries to do that for the SVE case.

Doing this regresses gcc.target/aarch64/sve/fabd_1.c.  I've added
-fno-trapping-math for now and filed PR108571 to track it.
A similar problem applies to fsubr_1.c.

I think this is likely to regress Power 10, since conditional
operations are only available for masked loops.  I think we'll
need to add -fno-trapping-math to any affected testcases,
but I don't have a Power 10 system to test on.

gcc/
	PR tree-optimization/96373
	PR tree-optimization/108979
	* tree-vect-stmts.cc (vectorizable_operation): Predicate trapping
	operations on the loop mask.  Reject partial vectors if this isn't
	possible.  Don't mask operations on invariants.

gcc/testsuite/
	PR tree-optimization/96373
	PR tree-optimization/108571
	PR tree-optimization/108979
	* gcc.target/aarch64/sve/fabd_1.c: Add -fno-trapping-math.
	* gcc.target/aarch64/sve/fsubr_1.c: Likewise.
	* gcc.target/aarch64/sve/fmul_1.c: Expect predicate ops.
	* gcc.target/aarch64/sve/fp_arith_1.c: Likewise.
	* gfortran.dg/vect/pr108979.f90: New test.