Don't re-allocate %esi on x86.

Recent changes have freed up %esi for general use on x86 when it is
not being used for R1.  However, x86 has a non-uniform register
architecture where there is no 8-bit equivalent of %esi.  The register
allocators aren't sophisticated enough to cope with this, so we have
to back off and treat %esi as non-allocatable for now.  (of course,
LLVM doesn't suffer from this problem)

One workaround would be to change the calling convention to use %rbx
for R1, however we can't change the calling convention now without
patching LLVM too.

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