Fixed another assert exposed by fuzzing. The utility function getRegisterEnum()

was asserting because the (RegClass, RegNum) combination doesn't make sense from
an encoding point of view.

Since getRegisterEnum() is used all over the place, to change the code to check
for encoding error after each call would not only bloat the code, but also make
it less readable.  An Err flag is added to the ARMBasicMCBuilder where a client
can set a non-zero value to indicate some kind of error condition while building
up the MCInst.  ARMBasicMCBuilder::BuildIt() checks this flag and returns false
if a non-zero value is detected.

llvm-svn: 101290