[coroutines] Get an IntegerType from the value instead of defaulting to 64 bit

This AliasPtr is being created always from an Int64 even for targets
where 32 bit is the proper type. e.g. “thumbv7-none-linux-android16”.
This causes the assert in the `get` func to fail as we're getting a 32
bit from the APInt.

Fix this by simply always just getting the type from the value instead.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D123272