Reapply "blockfreq: Rewrite BlockFrequencyInfoImpl"

This reverts commit r206677, reapplying my BlockFrequencyInfo rewrite.

I've done a careful audit, added some asserts, and fixed a couple of
bugs (unfortunately, they were in unlikely code paths).  There's a small
chance that this will appease the failing bots [1][2].  (If so, great!)

If not, I have a follow-up commit ready that will temporarily add
-debug-only=block-freq to the two failing tests, allowing me to compare
the code path between what the failing bots and what my machines (and
the rest of the bots) are doing.  Once I've triggered those builds, I'll
revert both commits so the bots go green again.

[1]: http://bb.pgr.jp/builders/ninja-x64-msvc-RA-centos6/builds/1816
[2]: http://llvm-amd64.freebsd.your.org/b/builders/clang-i386-freebsd/builds/18445

<rdar://problem/14292693>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206704 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/XCore/llvm-intrinsics.ll

@@ -287,9 +287,8 @@ define void @Unwind1() {
 ; CHECKFP: .LBB{{[0-9_]+}}
 ; CHECKFP-NEXT: ldc r2, 40
 ; CHECKFP-NEXT: add r2, r10, r2
-; CHECKFP-NEXT: add r0, r2, r0
+; CHECKFP-NEXT: add r2, r2, r0
 ; CHECKFP-NEXT: mov r3, r1
-; CHECKFP-NEXT: mov r2, r0
 ; CHECKFP-NEXT: ldw r9, r10[4]
 ; CHECKFP-NEXT: ldw r8, r10[5]
 ; CHECKFP-NEXT: ldw r7, r10[6]
@@ -337,9 +336,8 @@ define void @Unwind1() {
 ; CHECK-NEXT: ldc r2, 36
 ; CHECK-NEXT: ldaw r3, sp[0]
 ; CHECK-NEXT: add r2, r3, r2
-; CHECK-NEXT: add r0, r2, r0
+; CHECK-NEXT: add r2, r2, r0
 ; CHECK-NEXT: mov r3, r1
-; CHECK-NEXT: mov r2, r0
 ; CHECK-NEXT: ldw r10, sp[2]
 ; CHECK-NEXT: ldw r9, sp[3]
 ; CHECK-NEXT: ldw r8, sp[4]