Make -frewrite-rules into a dynamic flag; off for -O0
Argubly rewrite rules should not fire with -O0, and it turns out that when compiling GHC.Base with -O0 we get a crash if the rewrite rules do fire (see Note [Scoping for Builtin rules] in PrelRules). So unless someone yells, rewrite rules are off with -O0. The new (now dynamic) flag is -frewrite rules (with -fno-rewrite-rules to disable) The old (static) flag -frules-off is gone.
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