[1.8>master] [MERGE #4213 @sigatrev] ARM64: change to assign bits
[1.8>master] [MERGE #4213 @sigatrev] ARM64: change to assign bits

Merge pull request #4213 from sigatrev:ubfx

second param of BITFIELD is number of bits.

mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore