[GlobalISel] Fix choice of instruction selector for AArch64 at -O0 with -global-isel=0

Commit rL347861 introduced an unintentional change in the behaviour when
compiling for AArch64 at -O0 with -global-isel=0. Previously, explicitly
disabling GlobalISel resulted in using FastISel but an updated condition
in the commit changed it to using SelectionDAG. The patch fixes this
condition and slightly better organizes the code that chooses the
instruction selector.

Fixes PR40131.

Differential Revision: https://reviews.llvm.org/D56266

llvm-svn: 350626