[DSLX:ir_conv] IR convert trailing match arms w/ multiple patterns.

[cdleary]

Previously we would give an IR conversion error, which was a vestige from where we needed the trailing arm to be a wildcard. Now with exhaustiveness we know the trailing match arm must be usable as a default case in the priority select that we lower to.

[cdleary]

Rebased this, PTAL.

[proppy]

@cdleary do we want to also update the existing examples to remove unnecessary trailing match clause w/ wildcards?

[cdleary]

@proppy Yeah I'd think so, can't think of a reason why not -- I did a quick search for .md that have _ => and nothing was jumping out at me, do you see some I should have updated but missed?

[proppy]

found at least one :)

xls/xls/examples/jpeg/idct_chen.x

Line 239 in 6f68726

_ => fail!("invalid_column_index", s32:0)

but I was also wondering if we should make this a failure/warning during conversion?

[cdleary]

found at least one :)

xls/xls/examples/jpeg/idct_chen.x

Line 239 in 6f68726

_ => fail!("invalid_column_index", s32:0)

but I was also wondering if we should make this a failure/warning during conversion?

@proppy This is a warning https://sourcegraph.com/github.com/google/xls/-/blob/xls/dslx/warning_kind.h?L45 already-exhaustive-match , it's just disabled by default so Google-internal codebase can do the global change that enables it by default. In my repo I have a toolchain-level config that enables it by default.