aarch64: Make aarch64_add_offset work with -ftrapv [PR99540]

aarch64: Make aarch64_add_offset work with -ftrapv [PR99540]

aarch64_add_offset uses expand_mult to multiply the SVE VL by an
out-of-range constant.  expand_mult takes an argument to indicate
whether the multiplication is signed or unsigned, but in this
context the multiplication is effectively signless and so the
choice seemed arbitrary.

However, one of the things that the signedness input does is
indicate whether signed overflow should be trapped for -ftrapv.
We don't want that here, so we must treat the multiplication
as unsigned.

gcc/
2021-03-23  Jakub Jelinek  <jakub@redhat.com>

	PR target/99540
	* config/aarch64/aarch64.c (aarch64_add_offset): Tell
	expand_mult to perform an unsigned rather than a signed
	multiplication.

gcc/testsuite/
2021-03-23  Richard Sandiford  <richard.sandiford@arm.com>

	PR target/99540
	* gcc.dg/vect/pr99540.c: New test.