[HW] Make `hw::getBitWidth` use std::optional vs -1

[teqdruid]

Convert circt::getBitWidth to return std::optional<uint64_t>. Also convert the BitWidthTypeInterface getBitWidth method to return the same instead of a signed version. Update the callsites. Where the callsites do not check for it, add an assertion. In cases where a bitwidth was inappropriately assumed (e.g. canonicalizers) but wasn't checked or asserted, update the code to bail out appropriately without making changes.

[teqdruid]

@seldridge we'll see how this goes...

[teqdruid]

In trying to use AI locally (via vscode), I came across some code which (seems to) simply assume that a bitwidth is defined. Also, some cases where things simply assert where they should just bail out (IIRC some canonicalizers).