[PR middle-end/20491] combine generates bad subregs

[Alexandre Oliva]

Combine doesn't ensure the subregs it generates are valid.  In most
cases, insn recog will reject the invalid subregs, or reload will
somehow make them fit, but if the constraint of the insn or the asm
operand is "X", this won't work, so I think we're better off ensuring
we don't ever introduce subregs of non-REGs.

Does this look like a reasonable change to make?  If so, ok to install
if bootstrap and regtest on amd64-linux-gnu and i686-pc-linux-gnu
succeed?

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-- 
Alexandre Oliva             http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}