[llvm] [Thumb2][ARMAsmParser] Fix processing of `t2STR_{PRE,POST}_imm` when changing to its concrete form (PR #116757)

[Oliver Stannard via llvm-commits]

ostannard wrote:

These adjacent cases in that switch statement look like they have the same bug, and should be fixed at the same time:
```
t2LDR_PRE_imm
t2LDR_POST_imm
t2LDRB_PRE_imm
t2LDRB_POST_imm
t2STRB_PRE_imm
t2STRB_POST_imm
t2LDRH_PRE_imm
t2LDRH_POST_imm
t2STRH_PRE_imm
t2STRH_POST_imm
t2LDRSB_PRE_imm
t2LDRSB_POST_imm
t2LDRSH_PRE_imm
t2LDRSH_POST_imm
```

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