Add constraint 'v' in LLVM IR for vector registers and 'vr' and 'vm' for
C/C++ inline asm constraints.
Description at the begin of this chapter
` The constraint codes are, in general, expected to behave the same way they do in GCC. LLVM’s support is often implemented on an ‘as-needed’ basis, to support C inline asm code which was supported by GCC. A mismatch in behavior between LLVM and GCC likely indicates a bug in LLVM.
I would suggest remove v, it's impossible to implement v and vr/vm on GCC site.