Subzero: Use register availability during lowering to improve the code.

tests_lit/llvm2ice_tests/phi.ll

 ; This tests some of the subtleties of Phi lowering.  In particular,
 ; it tests that it does the right thing when it tries to enable
 ; compare/branch fusing.
 
 ; RUN: %p2i -i %s --filetype=obj --disassemble --args -O2 --phi-edge-split=0 \
 ; RUN:   | FileCheck %s
 ; RUN: %p2i -i %s --filetype=obj --disassemble --args -O2 --phi-edge-split=1 \
 ; RUN:   | FileCheck --check-prefix=SPLIT %s
 
 define internal i32 @testPhi1(i32 %arg) {
@@ skipping 74 matching lines @@
 ; .LtestPhi3$exit:
 ;         mov     DWORD PTR [ecx+1000], eax
 ;         ret
 ;
 ; This is bad because the final store address is supposed to be the
 ; same as the load address in the loop, but it has clearly been
 ; over-optimized into a null pointer dereference.
 
 ; CHECK-LABEL: testPhi3
 ; CHECK: push [[EBX:.*]]
-; CHECK: mov {{.*}},DWORD PTR [esp
-; CHECK: mov
-; CHECK: mov {{.*}},DWORD PTR [[ADDR:.*0x3e8]]
+; CHECK: mov [[EAX:.*]],DWORD PTR [esp
+; CHECK: mov [[ECX:.*]],[[EAX]]
+;;; start of loop body
+; CHECK: mov [[EDX:.*]],[[ECX]]
+; CHECK: mov {{.*}},DWORD PTR [{{.*}}+0x3e8]
 ; CHECK: cmp {{.*}},0x0
 ; CHECK: jne
-; CHECK: mov DWORD PTR [[ADDR]]
+;;; start of epilog
+; CHECK: mov DWORD PTR {{.}}[[EDX]]+0x3e8],
 ; CHECK: pop [[EBX]]
 
 ; Test of "advanced phi lowering" with undef phi arg (integer vector).
 define internal <4 x i32> @test_split_undef_int_vec(<4 x i32> %arg, i32 %cond) {
 entry:
   %cmp = icmp eq i32 %cond, 0
   br i1 %cmp, label %eq, label %exit
 eq:
   br label %exit
 exit:
@@ skipping 37 matching lines @@
   %cmp = icmp eq i32 %cond, 0
   br i1 %cmp, label %eq, label %exit
 eq:
   br label %exit
 exit:
   %merge = phi float [ %arg, %entry ], [ undef, %eq ]
   ret float %merge
 }
 ; SPLIT-LABEL: test_split_undef_float_scalar
 ; SPLIT: movss {{.*}},DWORD PTR