RFC: bug in combine

[Dale Johannesen]

The following demonstrates a bug in combine
(x86 -mtune=pentiumpro -O2):

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The test of f->b comes out as

   testl  $1048512, 73(%eax)

This is wrong, because 4 bytes starting at 73 goes outside the original 
object and can
cause a page fault.  The change from referencing a word at offset 72 to 
offset 73
happens in make_extraction in combine, and I propose to fix it thus:

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Still testing, but I'm a bit concerned this is overkill.  Are there 
targets/situations where
this transformation is useful or even necessary?  Comments?