[Mips] Suppress "right shift by too large amount" warning

Visual C++ shows the "right shift by too large amount" warning if
`MipsELFReference` is instantiated for 32-bit target and `Elf_Rel_Impl::getType`
method has `unsigned char` return type. We can freely suppress the warning in
that case because MIPS 32-bit ABI does not pack multiple relocation types into
the single field `r_type` and the `MipsELFReference::_tag` should be always
zero in that case.

No functional changes.

llvm-svn: 233088