Matt Arsenault e3fd8f83a8 AMDGPU: Correctly expand f64 sqrt intrinsic
rocm-device-libs and llpc were avoiding using f64 sqrt
intrinsics in favor of their own expansions. Port the
expansion into the backend. Both of these users should be
updated to call the intrinsic instead.

The library and llpc expansions are slightly different.
llpc uses an ldexp to do the scale; the library uses a multiply.

Use ldexp to do the scale instead of the multiply.
I believe v_ldexp_f64 and v_mul_f64 are always the same number of
cycles, but it's cheaper to materialize the 32-bit integer constant
than the 64-bit double constant.

The libraries have another fast version of sqrt which will
be handled separately.

I am tempted to do this in an IR expansion instead. In the IR
we could take advantage of computeKnownFPClass to avoid
the 0-or-inf argument check.
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+==============================================================================+
| How to organize the lit tests                                                |
+==============================================================================+

- If you write a test for matching a single DAG opcode or intrinsic, it should
  go in a file called {opcode_name,intrinsic_name}.ll (e.g. fadd.ll)

- If you write a test that matches several DAG opcodes and checks for a single
  ISA instruction, then that test should go in a file called {ISA_name}.ll (e.g.
  bfi_int.ll

- For all other tests, use your best judgement for organizing tests and naming
  the files.

+==============================================================================+
| Naming conventions                                                           |
+==============================================================================+

- Use dash '-' and not underscore '_' to separate words in file names, unless
  the file is named after a DAG opcode or ISA instruction that has an
  underscore '_' in its name.