arm64: implement: {uqshl, sqshl, sqshlu} (scalar, imm) and fix two
bugs in the implementation of the (vector, imm) variants.


git-svn-id: svn://svn.valgrind.org/vex@2923 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Rename Iop_QSalN*, Iop_QShlN* and Iop_QShlN*S so as to more accurately
reflect what they actually do, which is a zero-fill shift left followed
by one of three flavours of saturation (S->S, U->U or S->U).


git-svn-id: svn://svn.valgrind.org/vex@2924 8f6e269a-dfd6-0310-a8e1-e2731360e62c
No functional change.  Remove commented out code copied from the
arm32 port, which is never going to get used.


git-svn-id: svn://svn.valgrind.org/vex@2925 8f6e269a-dfd6-0310-a8e1-e2731360e62c