support phi ranges for machine-level IR

Add iterator ranges for machine instruction phis, similar to the IR-level
phi ranges added in r303964. I updated a few places to use this. Besides
general code simplification, this change will allow removing a non-upstream
change from Swift's copy of LLVM (in a better way than my previous attempt
in http://reviews.llvm.org/D19080).

https://reviews.llvm.org/D41672

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321783 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed