[CFE] Cache calls in type inference that almost always creates a FunctionType

InstanceAccessTarget calls "base.getGetterTypeForMemberTarget" a lot;
this CL caches the result which is almost always a new FunctionType,
thus creating fewer FunctionTypes.
When compiling compile.dart ~29% fewer:
Before this CL, compiling compile.dart created a little over 217k
FunctionTypes (down in a previous CL from almost 263k); with this CL
"only" ~153.5k FunctionTypes are created.

Change-Id: Ib110ed5e0d152a3b86044f20dc50f0924a083f46
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/298601
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>