Clients of the evrp range analysis may not have initialized the SCEV
infrastructure, and in fact my not want to (DOM for example).

Yet inside both vr-values.c and gimple-ssa-evrp-analyze.c we have calls
into SCEV (that will fault/abort if SCEV is not properly initialized).

This patch allows clients of vr-values.c and gimple-ssa-evrp-analyze.c
to indicate if they want SCEV analysis.

Bootstrapped and regression tested by itself as well as with the DOM
patches to use EVRP analysis  (which test the "don't want SCEV path).

OK for the trunk?

Jeff