Change instcombine to use FoldPHIArgGEPIntoPHI to fold two operand PHIs

Change instcombine to use FoldPHIArgGEPIntoPHI to fold two operand PHIs
instead of using FoldPHIArgBinOpIntoPHI.  In addition to being more
obvious, this also fixes a problem where instcombine wouldn't merge two
phis that had different variable indices.  This prevented instcombine
from factoring big chunks of code in 403.gcc.  For example:

 insn_cuid.exit:
-       %tmp336 = load i32** @uid_cuid, align 4
-       %tmp337 = getelementptr %struct.rtx_def* %insn_addr.0.ph.i, i32 0, i32 3
-       %tmp338 = bitcast [1 x %struct.rtunion]* %tmp337 to i32*
-       %tmp339 = load i32* %tmp338, align 4
-       %tmp340 = getelementptr i32* %tmp336, i32 %tmp339
        br label %bb62

 bb61:
-       %tmp341 = load i32** @uid_cuid, align 4
-       %tmp342 = getelementptr %struct.rtx_def* %insn, i32 0, i32 3
-       %tmp343 = bitcast [1 x %struct.rtunion]* %tmp342 to i32*
-       %tmp344 = load i32* %tmp343, align 4
-       %tmp345 = getelementptr i32* %tmp341, i32 %tmp344
        br label %bb62

 bb62:
-       %iftmp.62.0.in = phi i32* [ %tmp345, %bb61 ], [ %tmp340, %insn_cuid.exit ]
+       %insn.pn2 = phi %struct.rtx_def* [ %insn, %bb61 ], [ %insn_addr.0.ph.i, %insn_cuid.exit ]
+       %tmp344.pn.in.in = getelementptr %struct.rtx_def* %insn.pn2, i32 0, i32 3
+       %tmp344.pn.in = bitcast [1 x %struct.rtunion]* %tmp344.pn.in.in to i32*
+       %tmp341.pn = load i32** @uid_cuid
+       %tmp344.pn = load i32* %tmp344.pn.in
+       %iftmp.62.0.in = getelementptr i32* %tmp341.pn, i32 %tmp344.pn
        %iftmp.62.0 = load i32* %iftmp.62.0.in

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60325 91177308-0d34-0410-b5e6-96231b3b80d8