Revert commit causing #18619

("When compiling gshared code, always use the cached
isinst/castclass as it's faster than the cached one.")

This reverts commit e3866aa09217c5fe86ef9fe8d3bba3a618bc3ec5.

(See https://bugzilla.xamarin.com/show_bug.cgi?id=18619)

Change-Id: I2616a41cb0fe3838c1b2f1a39c72edae4c2c9063