[IndVars] Expand icmp in preheader rather than in loop

The motivation is that 'createInvariantCond' unconditionally
builds icmp in the loop block,  while it could always do it
in preheader. Build it in preheader instead.

Patch by Aleksandr Popov!

Differential Revision: https://reviews.llvm.org/D141994
Reviewed By: nikic