[FastISel][X86] Use getTypeForExtReturn in GetReturnInfo. (#80803)

The comment and code here seems to match getTypeForExtReturn. The
history shows that at the time this code was added, similar code existed
in SelectionDAGBuilder. SelectionDAGBuiler code has since been
refactored into getTypeForExtReturn.

This patch makes FastISel match SelectionDAGBuilder.

The test changes are because X86 has customization of
getTypeForExtReturn. So now we only extend returns to i8.

Stumbled onto this difference by accident.