Streamline the API of salvageDebugInfoImpl (NFC)

This patch refactors / simplifies salvageDebugInfoImpl(). The goal
here is to simplify the implementation of coro::salvageDebugInfo() in
a followup patch.

  1. Change the return value to I.getOperand(0). Currently users of
     salvageDebugInfoImpl() assume that the first operand is
     I.getOperand(0). This patch makes this information explicit. A
     nice side-effect of this change is that it allows us to salvage
     expressions such as add i8 1, %a in the future.

  2. Factor out the creation of a DIExpression and return an array of
     DIExpression operations instead. This change allows users that
     call salvageDebugInfoImpl() in a loop to avoid the costly
     creation of temporary DIExpressions and to defer the creation of
     a DIExpression until the end.

This patch does not change any functionality.

rdar://80227769

Differential Revision: https://reviews.llvm.org/D107383