[LV] Remove unnecessary DoExtraAnalysis guard (silent bug)

canVectorize is only checking if the loop has a normalized pre-header if DoExtraAnalysis is true.
This doesn't make sense to me because reporting analysis information shouldn't alter legality
checks. This is probably the result of a last minute minor change before committing (?).

Patch by Diego Caballero.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D40973

llvm-svn: 321172