Revert "r226071 - [RegisterCoalescer] Remove copies to reserved registers"

Reverting this while I investigate some bad behavior this is causing. As a
possibly-related issue, adding -verify-machineinstrs to one of the test cases
now fails because of this change:

  llc test/CodeGen/X86/2009-02-12-DebugInfoVLA.ll -march=x86-64 -o - -verify-machineinstrs

*** Bad machine code: No instruction at def index ***
- function:    foo
- basic block: BB#0 return (0x10007e21f10) [0B;736B)
- liverange:   [128r,128d:9)[160r,160d:8)[176r,176d:7)[336r,336d:6)[464r,464d:5)[480r,480d:4)[624r,624d:3)[752r,752d:2)[768r,768d:1)[78
4r,784d:0)  0@784r 1@768r 2@752r 3@624r 4@480r 5@464r 6@336r 7@176r 8@160r 9@128r
- register:    %DS
Valno #3 is defined at 624r

*** Bad machine code: Live segment doesn't end at a valid instruction ***
- function:    foo
- basic block: BB#0 return (0x10007e21f10) [0B;736B)
- liverange:   [128r,128d:9)[160r,160d:8)[176r,176d:7)[336r,336d:6)[464r,464d:5)[480r,480d:4)[624r,624d:3)[752r,752d:2)[768r,768d:1)[78
4r,784d:0)  0@784r 1@768r 2@752r 3@624r 4@480r 5@464r 6@336r 7@176r 8@160r 9@128r
- register:    %DS
[624r,624d:3)
LLVM ERROR: Found 2 machine code errors.

where 624r corresponds exactly to the interval combining change:

624B    %RSP<def> = COPY %vreg16; GR64:%vreg16
        Considering merging %vreg16 with %RSP
                RHS = %vreg16 [608r,624r:0)  0@608r
                updated: 608B   %RSP<def> = MOV64rm <fi#3>, 1, %noreg, 0, %noreg; mem:LD8[%saved_stack.1]
        Success: %vreg16 -> %RSP
        Result = %RSP

llvm-svn: 226086