Fix a nondeterminism in the ARM assembler.

Fix a nondeterminism in the ARM assembler.

Adding arbitrary records to ARM.td would break
basic-arm-instructions.s because selection of nop vs mov r0,r0 was
ambiguous (this will be tested by a subsequent addition to ARM.td).
An imperfect but sensible fix is to give precedence to match rules
that have more constraints.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162824 91177308-0d34-0410-b5e6-96231b3b80d8