MipsDelaySlotFiller: Don't move BUNDLE instructions into the delay slot

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). This sequence is created in MipsExpandPseudo and we use
finalizeBundle() to create the BUNDLE instruction.

However, the delay slot filler currently breaks this pattern since the BUNDLE
will be removed and so all instructions are moved into the delay slot.
Since the delay slot only executes the first instruction, this results in
incorrect computations (and run-time crashes) if the branch is taken.

The original test cases uses CHERI instructions, so for the test case here
I simple filled a BUNDLE with a no-op DADDiu $sp_64, -16 and DADDiu $sp_64, 16.

Reviewers: atanasyan

Reviewed By: atanasyan

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70944