[X86] Missed fold of `LEA + constant add` into `LEA` displacement

https://godbolt.org/z/hW16s9Mxh

GCC folds `x + y + constant` into a single `lea` (`lea eax, [rdi + 1 + rsi]`) using the displacement field, while LLVM currently emits an additional arithmetic instruction for these cases (`lea eax, [rdi + rsi] \ inc eax`).

Start with the linked Godbolt example and reproduce the X86 case where LLVM emits an LEA followed by an increment. Trace the instruction-combining path responsible for LEA plus constant-add folding. Done means LLVM emits a single LEA using its displacement field for the reported pattern.

Written by the indexing model from the issue text.