Completely rewrite libcall insertion by the legalizer, providing the

following handy-dandy properties:

1. it is always correct now
2. it is much faster than before
3. it is easier to understand

This implementation builds off of the recent simplifications of the
legalizer that made it single-pass instead of iterative.

This fixes JM/lencod, JM/ldecod, and
CodeGen/Generic/2006-02-12-InsertLibcall.ll (at least on PPC).

llvm-svn: 26144