[SimplifyLibCalls] New transformation: tan(atan(x)) -> x

This is enabled only under -ffast-math.
So, instead of emitting:
  4007b0:       50                      push   %rax
  4007b1:       e8 8a fd ff ff          callq  400540 <atanf@plt>
  4007b6:       58                      pop    %rax
  4007b7:       e9 94 fd ff ff          jmpq   400550 <tanf@plt>
  4007bc:       0f 1f 40 00             nopl   0x0(%rax)

for:
float mytan(float x) {
  return tanf(atanf(x));
}
we emit a single retq.

Differential Revision:	 http://reviews.llvm.org/D14302

llvm-svn: 252098