arm: Fix encoding of MVE vqshr[u]n

As it stands, these insns are incorrectly encoded as vqrshr[u]n.
Concretely, the problem can be seen as follows:

$ cat t.s
vqrshrnb.s16 q0,q0,#8
vqshrnb.s16 q0,q0,#8
$ gas/as-new t.s -march=armv8.1-m.main+mve -o t.o
$ binutils/objdump -d t.o -m armv8.1-m.main

t.o:     file format elf32-littlearm

Disassembly of section .text:

00000000 <.text>:
   0:   ee88 0f41       vqrshrnb.s16    q0, q0, #0
   4:   ee88 0f41       vqrshrnb.s16    q0, q0, #0

Here we assemble these two instructions to the same opcode.  The
encoding of the first is the correct, while the encoding of the second
is incorrect, and the bottom bit should be clear, see the Armv8-M ARM:
https://developer.arm.com/documentation/ddi0553/latest/

There is an additional problem here in that the disassembly of the
immediate is incorrect.  llvm-objdump shows the correct disassembly
here:

t.o:    file format elf32-littlearm

Disassembly of section .text:

00000000 <$t>:
       0: ee88 0f41     vqrshrnb.s16    q0, q0, #8
       4: ee88 0f41     vqrshrnb.s16    q0, q0, #8

Note that we defer adding a test for the correct encoding of these insns
until the next patch which fixes the disassembly issue.

gas/config/tc-arm.c

@@ -26621,10 +26621,10 @@ static const struct asm_opcode insns[] =
  mCEF(vshrnb,	  _vshrnb,	3, (RMQ, RMQ, I32z),	mve_vshrn),
  mCEF(vrshrnt,	  _vrshrnt,	3, (RMQ, RMQ, I32z),	mve_vshrn),
  mCEF(vrshrnb,	  _vrshrnb,	3, (RMQ, RMQ, I32z),	mve_vshrn),
- mCEF(vqshrnt,	  _vqrshrnt,	3, (RMQ, RMQ, I32z),	mve_vshrn),
- mCEF(vqshrnb,	  _vqrshrnb,	3, (RMQ, RMQ, I32z),	mve_vshrn),
- mCEF(vqshrunt,	  _vqrshrunt,	3, (RMQ, RMQ, I32z),	mve_vshrn),
- mCEF(vqshrunb,	  _vqrshrunb,	3, (RMQ, RMQ, I32z),	mve_vshrn),
+ mCEF(vqshrnt,	  _vqshrnt,	3, (RMQ, RMQ, I32z),	mve_vshrn),
+ mCEF(vqshrnb,	  _vqshrnb,	3, (RMQ, RMQ, I32z),	mve_vshrn),
+ mCEF(vqshrunt,	  _vqshrunt,	3, (RMQ, RMQ, I32z),	mve_vshrn),
+ mCEF(vqshrunb,	  _vqshrunb,	3, (RMQ, RMQ, I32z),	mve_vshrn),
  mCEF(vqrshrnt,	  _vqrshrnt,	3, (RMQ, RMQ, I32z),	mve_vshrn),
  mCEF(vqrshrnb,	  _vqrshrnb,	3, (RMQ, RMQ, I32z),	mve_vshrn),
  mCEF(vqrshrunt,  _vqrshrunt,	3, (RMQ, RMQ, I32z),	mve_vshrn),