[mlir][tosa] Remove 'Pure' trait from operations that are not speculatable (#185700)

This commit removes the 'Pure' trait from a number of TOSA operations.
Instead of marking most ops as pure by default, the trait is now opt-in
for operations that are provably side-effect free and speculatable.

Several operations were previously marked as pure unintentionally.

The following operations have had 'Pure' removed (reason in brackets):
- ARGMAX (out-of-range index)
- AVG_POOL2D (accumulator overflow/underflow)
- AVG_POOL2D_ADAPTIVE (same as above)
- CONV2D (accumulator overflow/underflow)
- CONV2D_BLOCK_SCALED (accumulator overflow/underflow)
- CONV3D (accumulator overflow/underflow)
- DEPTHWISE_CONV2D (accumulator overflow/underflow)
- MATMUL (accumulator overflow/underflow)
- MATMUL_T_BLOCK_SCALED (accumulator overflow/underflow)
- TRANSPOSE_CONV2D (accumulator overflow/underflow)
- ADD (overflow)
- SUB (underflow)
- MUL (invalid shift, overflow)
- ARITHMETIC_RIGHT_SHIFT (invalid shift value)
- LOGICAL_LEFT_SHIFT (invalid shift value)
- LOGICAL_RIGHT_SHIFT (invalid shift value)
- INTDIV (division by zero)
- POW (negative exponent restrictions)
- TABLE (invalid slope computation)
- ABS (underflow)
- NEGATE (overflow/underflow)
- REDUCE_PRODUCT (overflow)
- REDUCE_SUM (overflow)
- GATHER (out-of-range indices)
- SCATTER (out-of-range or duplicate indices)
- RESCALE (overflow/underflow)

Many of these operations can exhibit undefined behaviour when a
`REQUIRE` condition in the TOSA specification pseudocode fails. Whether
such failures result in a runtime error is implementation-defined. As a
result, speculating or reordering these operations can change program
behaviour.

For this reason, the `AlwaysSpeculatable` property implied by `Pure` is
not valid for these ops. The `NoMemoryEffect` trait is retained, as
these operations do not have direct memory side effects.