[LegalizeVectorTypes] Allow illegal indices when splitting extract_vector_elt

Summary:
Fixes PR40267, in which the removed assertion was triggering on
perfectly valid IR. As far as I can tell, constant out of bounds
indices should be allowed when splitting extract_vector_elt, since
they will simply be propagated as out of bounds indices in the
resulting split vector and handled appropriately elsewhere.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya

Differential Revision: https://reviews.llvm.org/D57471

llvm-svn: 352702