ir3/lower_io_offsets: Try propagate SSBO's SHR into a previous shift instruction

While we lack value range tracking, this patch tries to 'manually' propogate
the division by 4 to calculate SSBO element-offset, into a possible previous
shift operation (shift left or right); checking that it is safe to do so.

This should help in cases like ie. when accessing a field in an array of
structs, where the offset is likely defined as base plus a multiplication
by a struct or array element size.

See dEQP test 'dEQP-GLES31.functional.ssbo.atomic.xor.highp_uint'
for an example of a shader that benefits from this.

Reviewed-by: Rob Clark <[email protected]>

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