rs6000: Merge lshrsi3 and lshrdi3

rs6000: Merge lshrsi3 and lshrdi3

For this create a new mode_attr "hH".

Also change "i" constraints on the shift amount to "n", which better
describes what it really is (GCC takes the integer value of these
operands and does arithmetic on them; symbolic constants will not work
here).

Also merge the "dot" insns with the corresponding splitters.  To do
this, don't allow the dot insns for CBE non-microcode mode at all
(it previously would just split it back always).

From-SVN: r211876