Allow a register to be redefined multiple times in a basic block.

LiveVariableAnalysis was a bit picky about a register only being redefined once,
but that really isn't necessary.

Here is an example of chained INSERT_SUBREGs that we can handle now:

68      %reg1040<def> = INSERT_SUBREG %reg1040, %reg1028<kill>, 14
                register: %reg1040 +[70,134:0)
76      %reg1040<def> = INSERT_SUBREG %reg1040, %reg1029<kill>, 13
                register: %reg1040 replace range with [70,78:1) RESULT: %reg1040,0.000000e+00 = [70,78:1)[78,134:0)  0@78-(134) 1@70-(78)
84      %reg1040<def> = INSERT_SUBREG %reg1040, %reg1030<kill>, 12
                register: %reg1040 replace range with [78,86:2) RESULT: %reg1040,0.000000e+00 = [70,78:1)[78,86:2)[86,134:0)  0@86-(134) 1@70-(78) 2@78-(86)
92      %reg1040<def> = INSERT_SUBREG %reg1040, %reg1031<kill>, 11
                register: %reg1040 replace range with [86,94:3) RESULT: %reg1040,0.000000e+00 = [70,78:1)[78,86:2)[86,94:3)[94,134:0)  0@94-(134) 1@70-(78) 2@78-(86) 3@86-(94)

rdar://problem/8096390

llvm-svn: 106152