Add INLINE pragmas for Enum helper methods

As #21343 showed, we need to be super-certain that the "helper
methods" for Enum instances are actually inlined or specialised.

I also tripped over this when I discovered that numericEnumFromTo
and friends had no pragmas at all, so their performance was very
fragile.  If they weren't inlined, all bets were off.  So I've added
INLINE pragmas for them too.

See new Note [Inline Enum method helpers] in GHC.Enum.

I also expanded Note [Checking for INLINE loop breakers] in
GHC.Core.Lint to explain why an INLINE function might temporarily
be a loop breaker -- this was the initial bug report in #21343.

Strangely we get a 16% runtime allocation decrease in
perf/should_run/T15185, but only on i386.  Since it moves in the right
direction I'm disinclined to investigate, so I'll accept it.

Metric Decrease:
    T15185