Handle BUNDLE instructions in MipsAsmPrinter

Summary:
In our CHERI fork we use BUNDLE instructions to ensure that a
three-instruction sequence to generate a program-counter-relative value is
emitted without reordering or insertions (since that would break the 32-bit
offset computation).

Currently MipsAsmPrinter asserts when it encounters a pseudo instruction.
To handle BUNDLE we can simply skip the instruction which will then make
EmitInstruction() process the contents of the bundle in order.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: merge_guards_bot, sdardis, hiraditya, jrtc27, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70945