Prefix IR dumps with LiveInterval indices when possible. This turns

Prefix IR dumps with LiveInterval indices when possible.  This turns
this:

	%ESI<def> = MOV32rr %EDI<kill>
	ADJCALLSTACKDOWN64 0, %RSP<imp-def>, %EFLAGS<imp-def,dead>, %RSP<imp-use>
	%reg1027<def> = MOVZX64rr32 %ESI
	%reg1027<def> = ADD64ri8 %reg1027, 15, %EFLAGS<imp-def,dead>
	%reg1027<def> = AND64ri8 %reg1027, -16, %EFLAGS<imp-def,dead>
	%RDI<def> = MOV64rr %RSP
	%RDI<def> = SUB64rr %RDI, %reg1027<kill>, %EFLAGS<imp-def,dead>
	%RSP<def> = MOV64rr %RDI

into this:

4	%reg1024<def> = MOV32rr %EDI<kill>
12	ADJCALLSTACKDOWN64 0, %RSP<imp-def>, %EFLAGS<imp-def,dead>, %RSP<imp-use>
20	%reg1025<def> = MOVZX64rr32 %reg1024
28	%reg1026<def> = MOV64rr %reg1025<kill>
36	%reg1026<def> = ADD64ri8 %reg1026, 15, %EFLAGS<imp-def,dead>
44	%reg1027<def> = MOV64rr %reg1026<kill>
52	%reg1027<def> = AND64ri8 %reg1027, -16, %EFLAGS<imp-def,dead>
60	%reg1028<def> = MOV64rr %RSP
68	%reg1029<def> = MOV64rr %reg1028<kill>
76	%reg1029<def> = SUB64rr %reg1029, %reg1027<kill>, %EFLAGS<imp-def,dead>
84	%RSP<def> = MOV64rr %reg1029

This helps greatly when debugging register allocation and coalescing
problems.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@76615 91177308-0d34-0410-b5e6-96231b3b80d8