[patch] Use hard_reg_set_iterator in a few places

[Steven Bosscher]

Hello,

GCC has hard_reg_set_iterator to iterate quickly over a HARD_REG_SET,
but it's not used a lot. Attached patch makes a few files use it to
iterate over regs_invalidated_by_call. If this is OK, I'd like to
convert loops over HARD_REG_SETs to iterators where possible.

Bootstrapped&tested on x86_64-unknown-linux-gnu. OK for trunk?

Ciao!
Steven