[Clang] refactor CodeGenFunction::EmitAsmStmt NFC

Prerequisite to further modifications in D136497.

Basically, there is a large body of code in CodeGenFunction::EmitAsmStmt
for emitting stores of outputs. We want to be able to repeat this logic,
for each destination of a callbr (rather than just the default
destination which is what the code currently does).

Also does some smaller cleanups like whitespace cleanups, and removing
pointless casts.

Reviewed By: void, jyknight

Differential Revision: https://reviews.llvm.org/D137113