[AArch64][GlobalISel] Enable GlobalISel at -O0 by default

Tests updated to explicitly use fast-isel at -O0 instead of implicitly.

This change also allows an explicit -fast-isel option to override an
implicitly enabled global-isel. Otherwise -fast-isel would have no effect at -O0.

Differential Revision: https://reviews.llvm.org/D41362

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321655 91177308-0d34-0410-b5e6-96231b3b80d8
44 files changed