Bug 417452 - s390x: Force 12-bit amode for vector stores in isel

Bug 417452 - s390x: Force 12-bit amode for vector stores in isel

It was seen that the s390 instruction selector chose a wrong addressing
mode for storing a vector register.  The VST instruction only handles
short (12-bit unsigned) displacements, but a long (20-bit signed)
displacement was generated instead, resulting in a panic:

vex: the `impossible' happened:
   s390_insn_store_emit: unknown dst->tag for HRcVec128

The fix prevents long displacements for vector store operations.  It also
optimizes vector store operations from an Iex_Get, by converting them to a
memory copy.  This optimization was already performed for integer
registers.