[GlobalISel] Make EnableGlobalISel always set when GISel is enabled

Change meaning of TargetOptions::EnableGlobalISel. The flag was
previously set only when a target switched on GlobalISel but it is now
always set when the GlobalISel pipeline is enabled. This makes the flag
consistent with TargetOptions::EnableFastISel and allows its use in
other parts of the compiler to determine when GlobalISel is enabled.

The EnableGlobalISel flag had previouly only one use in
TargetPassConfig::isGlobalISelAbortEnabled(). The method used its value
to determine if GlobalISel was enabled by a target and returned false in
such a case. To preserve the current behaviour, a new flag
TargetOptions::GlobalISelAbort is introduced to separately record the
abort behaviour.

Differential Revision: https://reviews.llvm.org/D54518

llvm-svn: 347861