Revert -r278269 [IndVarSimplify] Eliminate zext of a signed IV when the IV is...

Revert -r278269 [IndVarSimplify] Eliminate zext of a signed IV when the IV is known to be non-negative

This change needs to be reverted in order to revert -r278267 which cause performance regression on MultiSource/Benchmarks/TSVC/Symbolics-flt/Symbolics-flt from LNT and some other bechmarks.

See comments on https://reviews.llvm.org/D18777 for details.

llvm-svn: 279432