Rewrite fast-isel integer cast handling to handle more cases, and to be...

Rewrite fast-isel integer cast handling to handle more cases, and to be simpler and more consistent.

The practical effects here are that x86-64 fast-isel can now handle trunc from i8 to i1, and ARM fast-isel can handle many more constructs involving integers narrower than 32 bits (including loads, stores, and many integer casts).

rdar://9437928 .

llvm-svn: 132099