Update Pop text format to handle tuples

[tlively]

Previously Pops were printed as ({type}.pop), and if the popped type
was a tuple, something like ((i32, i64).pop) would get
printed. However, the parser didn't support pops of anything besides
single basic types.

This PR changes the text format to be (pop *) and adds support
for parsing pops of tuples of basic types. The text format change is
designed to make parsing simpler. This change is necessary for writing
Poppy IR tests that contain break or return instructions that consume
multiple values (see #3059).

[aheejin]

LGTM.

This change is necessary for writing Poppy IR tests that contain break or return instructions that consume multiple values (see #3059).

Why is this? I checked #3059 but am not sure. Can you elaborate?

[tlively]

This change is necessary for writing Poppy IR tests that contain break or return instructions that consume multiple values (see #3059).

Why is this? I checked #3059 but am not sure. Can you elaborate?

Return and Break instruction classes only take a single child, so that child has to have a tuple type if the function or break target expects multiple values. In Poppy IR, that means the child has to be a tuple-typed pop, so it is useful to be able to write those in the text format. I'll elaborate in the commit message as well.