Add a few missing HOST_WIDE_INT casts.

Rename references to `significant' bits to be `nonzero' bits;
specifically the following, but also local vars and comments:
(reg_nonzero_bits): Renamed from reg_significant.
(nonzero_bits): Renamed from significant_bits.
(nonzero_sign_valid): Renamed from significant_valid.
(set_nonzero_bits_and_sign_copies): Renamed from set_significant.

From-SVN: r2975