[mips][ias] Fold needsExpansion() and expandInstruction() together. NFC.

Summary:
Previously we maintained two separate switch statements that had to be kept in
sync. This patch merges them into a single switch.

Reviewers: vkalintiris

Subscribers: llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D14012

llvm-svn: 251369