Fix a minor bug in previous checking

Fix a minor bug in previous checking
Enable folding of long seteq/setne comparisons into branches and select instructions
Implement unfolded long relational comparisons against a constants a bit more efficiently

Folding comparisons changes code that looks like this:
        mov %EAX, DWORD PTR [%ESP + 4]
        mov %EDX, DWORD PTR [%ESP + 8]
        mov %ECX, %EAX
        or %ECX, %EDX
        sete %CL
        test %CL, %CL
        je .LBB2 # PC rel: F

into code that looks like this:
        mov %EAX, DWORD PTR [%ESP + 4]
        mov %EDX, DWORD PTR [%ESP + 8]
        mov %ECX, %EAX
        or %ECX, %EDX
        jne .LBB2 # PC rel: F

This speeds up 186.crafty by 6% with llc-ls.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12702 91177308-0d34-0410-b5e6-96231b3b80d8