[llvm] [Thumb2][ARMAsmParser] Fix processing of `t2STR_POST_imm` when changing to its concrete form (PR #116757)

[David Green via llvm-commits]

davemgreen wrote:

Can we make sure that pre-inc works at the same time?
https://godbolt.org/z/YPrxcGsjf

https://github.com/llvm/llvm-project/pull/116757