Using VRP to simplify conditionals

[Jeffrey A Law <law at redhat dot com>]

In my ongoing quest to remove the VRP bits from DOM, this patch
enhances tree-vrp.c so that it can simplify a conditional using
range information, even when the conditional is not a compile-time
constant.

For example, if we have an object with the range [3..INF] and we
have a test obj <= 3, then we can simplify the test to obj == 3.

This can generate slightly better code and allows us to propagate
the value 3 for the object when the condition is true, which can
lead to some further simplifications.

This patch also removes the distinct IL walk to simplify statements
using range information and instead just calls back into the VRP
bits during the IL walk in substitute_and_fold.

Overall we get ever so slightly better code and a slightly faster
compiler.

We can't quite remove the equivalent code from DOM yet -- we'd need
to run another VRP pass later in the SSA optimization path to 
pick up the numerous secondary optimization opportunities.  The
VRP + redundancy elimination + VRP is still currently a significant
compile-time hit.

Bootstrapped and regression tested on i686-pc-linux-gnu.

Jeff

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