[IndVars] Assert that the incoming IR is in LCSSA

Since we already assert that the outgoing IR is in LCSSA, it is easy to
get misled into thinking that -indvars broke LCSSA if the incoming IR is
non-LCSSA.  Checking this pre-condition will make such cases break in
more obvious ways.

Inspired by (but does _not_ fix) PR26682.

llvm-svn: 271196