Update stale register name comments in filter_fir_s32.S

[Copilot]

Comments in filter_fir_s32.S still reference old architectural register names (x18, x19) after the codebase transitioned to ABI names (s2, s3).

Changes:

• Update comments at lines 166-167 and 174 to use ABI register names (s2, s3) instead of architectural names (x18, x19)

The code itself is correct—only documentation is stale:

// Before:
// (from the block above):  x19 = 1, x18 = 1<<(a2 - 1)

// After:
// (from the block above):  s3 = 1, s2 = 1<<(a2 - 1)

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[andrewstanfordjason]

this is pointless!