fix: local args

[talubik]

Fix __local argument pointer generation in pocl Vortex device

When I tried to run OpenCL kernels with __local memory arguments, I found that the generated argument pointers were incorrect. This Pull Request fixes two bugs in the LLVM IR generation for __local kernel arguments.

Bug 1: ArgOffset not updated before reading per-argument offset

When processing the first __local argument, ArgOffset was captured before reading __local_size from the argument buffer. After __local_size is read, arg_offset advances by 4 bytes, but the cached ArgOffset constant is never updated. As a result, the per-argument offset field is read from the same position as __local_size, returning the total local memory size instead of the actual offset (which should be 0 for the first argument).

Bug 2: I8PtrTy used instead of I8Ty in GEP

The GEP instruction that applies the per-argument offset to the local memory base pointer used I8PtrTy (pointer type, 8 bytes on 64-bit) as the element type instead of I8Ty (1 byte). This caused the offset to be scaled by 8, placing the pointer far past the correct location in the allocated local memory block.

Effect of each bug (local_size = 4, B - A expected = 4)

Bugs present	B - A result
Both bugs	-32
Bug 2 only (I8PtrTy)	+32
Bug 1 only (ArgOffset)	-4
Fixed (this PR)	4 ✓

I also added a test which verifies the correct pointer layout of two __local arguments by checking that B - A == local_work_size. The test successfully passes with this fix and fails under all three buggy configurations.