Fix for equivalent register handling

[Jeff Law <law at redhat dot com>]

 Reload has the ability to replace a pseudo with its equivalent memory 
location.  This is fine and good.

Imagine:

  1. We have a pseudo (call is pseudo A) with a read-only memory 
equivalent.  Pseudo A does not get a hard reg

  2. Pseudo A crosses a call (because the memory is readonly, we will 
not invalidate the equivalency)

  3. The equivalent memory address references another pseudo (call it 
pseudo B)

  4. Pseudo B does not cross calls and is assigned a call-clobbered 
hard reg.

  5. reload replaces pseudo A with its equivalent memory form and in 
doing so lengthens the lifetime of pseudo B and causes pseudo B to be 
live across a call.

Obviously this is bad.

This patch removes the special case code which avoided invalidating 
certain memory equivalences for const/pure calls.

Bootstrapped and regression tested on x86_64; I also verified this fixed 
the testsuite regression seen with the range splitting code on x86_64.

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