[FIRRTL] Allow arbitrary LowerMemory dedup

[seldridge]

Remove the restriction on allowing design/ex-design deduplication of
memories in FIRRTL's LowerMemory pass. This is done to avoid problems
with unpredictable naming. It is unclear why this restriction was put on
the pass originally.

Assisted-by: pi.dev:claude-opus-4-8

[circt-bot]

Results of circt-tests run for a6eea91 compared to results for ab9b399:

sv-tests

Changes in emitted diagnostics:

• +2 total change
• +1 error: invalid operands to binary expression ('string' and 'bit[15:0]')
• +1 error: invalid operands to binary expression ('string' and 'bit[7:0]')