bpf: accept all asm register names

Sometimes when people compile bpf programs with
"clang ... -target bpf ...", the kernel header
files may contain host arch inline assembly codes
as in the patch https://patchwork.kernel.org/patch/10119683/
by Arnaldo Carvaldo de Melo.

The current workaround in the above patch
is to guard the inline assembly with "#ifndef __BPF__"
marco. So when __BPF__ is defined, these macros will
have no use.

Such a method is not extensible. As a matter of fact,
most of these inline assembly codes will be thrown away
at the end of clang compilation.

So for bpf target, this patch accepts all asm register
names in clang AST stage. The name will be checked
again during llc code generation if the inline assembly
code is indeed for bpf programs.

With this patch, the above "#ifndef __BPF__" is not needed
any more in https://patchwork.kernel.org/patch/10119683/

.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 329823