fix the bug that for block_k=16 mma, the compilation crash on Ampere.

[bingyizh233]

The origin issue is reported here: triton-lang#3435 The issue happens during compilation, when arith.sitofp (from i8 to fp16) operates on the tensor operand which has dot_op layout with the first dimension of the tensor being 16 and opidx = 1.
For example: %104 = arith.sitofp %103 : tensor<16x64xi8, #triton_gpu.dot_op<{opIdx = 1, parent = #mma, kWidth = 4}>> to tensor<16x64xf16, #triton_gpu.dot_op<{opIdx = 1, parent = #mma, kWidth = 4}>>

Investigation shows that the bug happens in TritonGPUToLLVM pass. in the corner case (block_k = 16 and opidx = 1) extra elements will be unpacked in include/triton/Conversion/TritonGPUToLLVM/ElementwiseOpToLLVM.h:line 186-194. The code unpacks extra elements due to an implicit assumption in lib/Dialect/TritonGPU/IR/Dialect.h, at line 2000, at least 4 rep (e.g., i32) will be loaded.

Therefore, in our patch, extra loaded elements are dropped in the corner case (block_k = 16 and opidx = 1).

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[gflegar]

For tracking: this has been opened as a triton-lang#4768 upstream