s390x: Fix VLRL and VSTRL insns (Bug 498422)

s390x: Fix VLRL and VSTRL insns   (Bug 498422)

The replacement field for these insns is 12-bit wide. So its value
cannot possibly be represented by a UChar. Use UShort instead.

This was found by disasm-test like so:

./disasm-test --run vlrl
*** mismatch VEX: |vlrl  %v6,255,4|    objdump: |vlrl  %v6,4095,4|

./disasm-test --run vstrl
*** mismatch VEX: |vstrl  %v6,255(%r4),7|   objdump: |vstrl  %v6,4095(%r4),7|

Fixes https://bugs.kde.org/show_bug.cgi?id=498422