[LegalizeVectorTypes] Allow illegal indices when splitting extract_vector_elt

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352702 91177308-0d34-0410-b5e6-96231b3b80d8