arm64 insn selector: improved handling of Or1/And1 trees.

arm64 insn selector: improved handling of Or1/And1 trees.

This is the exact analog of cadd90993504678607a4f95dfe5d1df5207c1eb0, to the
point of almost being a copy-n-paste.  That commit split (amd64) iselCondCode
into two functions, iselCondCode_C (existing) and iselCondCode_R (new).  The
latter computes an I1-typed expression into a register rather than a condition
code.  The two functions cooperate so as to minimise between conversions between
a condition-code value and a value in a register.