[LV] Avoid unnecessary IV scalar-to-vector-to-scalar conversions

This patch prevents increases in the number of instructions, pre-instcombine,
due to induction variable scalarization. An increase in instructions can lead
to an increase in the compile-time required to simplify the induction
variables. We now maintain a new map for scalarized induction variables to
prevent us from converting between the scalar and vector forms.

This patch should resolve compile-time regressions seen after r274627.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275419 91177308-0d34-0410-b5e6-96231b3b80d8