amd64 insn selector: improved handling of Or1/And1 trees.

amd64 insn selector: improved handling of Or1/And1 trees.

This splits function iselCondCode into iselCondCode_C and iselCondCode_R, the
former of which is the old one that computes boolean expressions into an amd64
condition code, but the latter being new, and computes boolean expressions
into the lowest bit of an integer register.  This enables much better code
generation for Or1/And1 trees, which now result quite commonly from the new
&&-recovery machinery in the front end.