[Target][ARM] Replace outdated getARMVPTBlockMask function

getARMVPTBlockMask was an outdated function that only handled basic
block masks: T, TT, TTT and TTTT. This worked fine before the MVE
VPT Block Insertion Pass improvements as it was the only kind of
masks that it could generate, but now it can generate more complex
masks that uses E predicates, so it's dangerous to use that function
to calculate VPT/VPST block masks.

I replaced it with 2 different functions:
  - expandPredBlockMask, in ARMBaseInfo. This adds an "E" or "T" at
    the end of an existing PredBlockMask.
  - recomputeVPTBlockMask, in Thumb2InstrInfo. This takes an iterator
    to a VPT/VPST instruction and recomputes its block mask by looking
    at the predicated instructions that follows it. This should be
    used to recompute a block mask after removing/adding a predicated
    instruction to the block.

The expandPredBlockMask function is pretty much imported from the MVE
VPT Blocks pass.

I had to change the ARMLowOverheadLoops and MVEVPTBlocks passes as well
so they could use these new functions.

Differential Revision: https://reviews.llvm.org/D78201

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