[PATCH] Implement Named Register Global Variables in LLVM

[Rafael Ávila de Espíndola]

Warning: There is no register reservation at the moment, and so far it
only works with the stack pointer on selected architectures (ARM, ARM64,
x86_64 and AArch64). Work to include a wider bank of registers is in
progress.

Please make it clear that work to include allocatable registers is
*not* under way. That is a much harder and brittler feature.  We need
a full discussion an llvmdev before claiming that we may ever support
it.

on the named-reg-alloc.ll tests, please include a fixme about
producing a more specific error message about allocatable registers.

http://reviews.llvm.org/D3261