[VectorUtils] Fix nasty use-after-free
In truncateToMinimalBitwidths() we were RAUW'ing an instruction then
erasing it. However, that intruction could be cached in the map we're
iterating over. The first check is "I->use_empty()" which in most
cases would return true, as the (deleted) object was RAUW'd first so
would have zero use count. However in some cases the object could
have been polluted or written over and this wouldn't be the case.
Also it makes valgrind, asan and traditionalists who don't like their
compiler to crash sad.
No testcase as there are no externally visible symptoms apart from a
crash if the stars align.
Fixes PR26509.
This should fix crashes when building a number of ports on arm64.
Reported by: andrew
87ea0ad898