[X86] Fix SP adjustment in stack probes emitted on 32-bit Windows.

Commit r306010 adjusted the condition as follows:

-  if (Is64Bit) {
+  if (!STI.isTargetWin32()) {

The intent was to preserve the behavior on all Windows platforms
but extend the behavior on 64-bit Windows platforms to every
other one. (Before r306010, emitStackProbeCall only ever executed
when emitting code for Windows triples.)

Unfortunately,
  if (Is64Bit && STI.isOSWindows())
is not the same as
  if (!STI.isTargetWin32())
because of the way isTargetWin32() is defined:

  bool isTargetWin32() const {
    return !In64BitMode && (isTargetCygMing() ||
                            isTargetKnownWindowsMSVC());
  }

In practice this broke the JIT tests on 32-bit Windows, which did not
satisfy the new condition:

    LLVM :: ExecutionEngine/MCJIT/2003-01-15-AlignmentTest.ll
    LLVM :: ExecutionEngine/MCJIT/2003-08-15-AllocaAssertion.ll
    LLVM :: ExecutionEngine/MCJIT/2003-08-23-RegisterAllocatePhysReg.ll
    LLVM :: ExecutionEngine/MCJIT/test-loadstore.ll
    LLVM :: ExecutionEngine/OrcMCJIT/2003-01-15-AlignmentTest.ll
    LLVM :: ExecutionEngine/OrcMCJIT/2003-08-15-AllocaAssertion.ll
    LLVM :: ExecutionEngine/OrcMCJIT/2003-08-23-RegisterAllocatePhysReg.ll
    LLVM :: ExecutionEngine/OrcMCJIT/test-loadstore.ll

because %esp was not updated correctly. The failures are only visible
on a MSVC 2017 Debug build, for which we do not have bots.

llvm-svn: 306142