[X86] Make hasFP constant time

[X86] Make hasFP constant time

We need a frame pointer if there is a push/pop sequence after the
prologue in order to unwind the stack.  Scanning the instructions to
figure out if this happened made hasFP not constant-time which is a
violation of expectations.  Let's compute this up-front and reuse that
computation when we need it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256730 91177308-0d34-0410-b5e6-96231b3b80d8