Fix a bug in insn selection. For some reason Iop_1UtoXYZ did no

Fix a bug in insn selection. For some reason Iop_1UtoXYZ did no
zero-extension. That is essential, as not all computation is donw
using 8-byte values.
For example
- do a 64-bit computation in r1; assume leftmost 4 bytes != 0
- do a 32-bit computation in r1; leftmost 4 bytes are untouched != 0
- do 32to1 on r1; rightmost 4 bytes == 1; leftmost 4 bytes != 0
- do 1Uto64 on r1
Without zero-extension r1 will contain a value that is not boolean

git-svn-id: svn://svn.valgrind.org/vex/trunk@2458