Callee-saved regs are not eliminated well when concatenated with LLIL_SPLIT_REG

Binary Ninja Version

2.4.2950-dev Personal

Describe the bug

binja-hexagon lifts allocframe by pushing LLIL_SPLIT_REG(LR, FP) onto the stack. And, it doesn't use LLIL_PUSH but manipulates SP directly. This confuses binja, and it cannot elide LR:FP from MLIL or HLIL.

00000028      int32_t FP
00000028      int32_t FP_1
00000028      int32_t LR
00000028      int32_t LR_1
00000028      LR_1:FP_1 = LR:FP
00000028      return 0x100

To Reproduce
Steps to reproduce the behavior:

1. Install https://github.com/google/binja-hexagon/ plugin
2. Open https://github.com/google/binja-hexagon/blob/main/test_binaries/prebuilt/bn_llil_test_app in binja
3. Navigate to test_allocframe function

Expected behavior

I want the HLIL to look like:

00000028      return 0x100

Version and Platform (required):

• Binary Ninja: 2.4.2950-dev Personal
• OS: Debian Linux
• Version Buster

Additional context

Split from https://github.com/google/binja-hexagon/issues/3

Install the binja-hexagon plugin, open its prebuilt bn_llil_test_app, and navigate to the test_allocframe function as described. Inspect how LLIL_SPLIT_REG(LR, FP), direct SP manipulation, and the resulting MLIL/HLIL are handled. Done means the redundant LR:FP assignment is eliminated and HLIL shows only return 0x100.