[PATCH] Improve error handling of x86 inline-asm constraint "=f"

[Akira Hatanaka]

This patch makes the following changes to improve error handling of x86
inline-asm constraint '=f'. According to the following link, it is a
mistake to use the 'f' constraint for output operands.

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

1. Override TargetLowering::ComputeConstraintToUse so that this error can
be detected during instruction selection. Currently, it's detected in the
x86 FP stackifier pass (X86FloatingPoint.cpp), which is a pass that is run
much later.

2. Terminate immediately after this error is detected. Currently, clang
continues compiling the program after seeing this error, because the
inline-asm diagnostic handler that is installed doesn't terminate upon
seeing this error. As an example of why this is bad, I attached a program
that exposes an internal error when compiled with clang. clang didn't stop
compiling until it hit a call to report_fatal_error.

*main.c:17:7: **error: **illegal "f" output constraint*

      ".intel_syntax\n"

*      ^*

*fatal error: *error in backend: Access past stack top!


<rdar://problem/17476689>
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