[llvm] [LLParser] Support identifiers like `qnan` and `pinf` for special FP values (PR #102790)

[Joshua Cranmer via llvm-commits]

================
@@ -4426,6 +4425,21 @@ represented by ``0xH`` followed by 4 hexadecimal digits. The bfloat 16-bit
 format is represented by ``0xR`` followed by 4 hexadecimal digits. All
 hexadecimal formats are big-endian (sign bit at the left).
 
+Some of the special floating point values can be represented by the following
+identifiers:
+
+    +-----------+---------------------------------------------------+
+    | Name      | Description                                       |
+    +===========+===================================================+
+    | ``qnan``  | Positive quiet NaN w/ payload equals to zero      |
+    +-----------+---------------------------------------------------+
+    | ``snan``  | Positive signaling NaN w/ payload equals to zero  |
----------------
jcranmer-intel wrote:

There is no sNaN with an all-0 payload, that would be an infinity instead--the payload needs to have some bit set to not be an infinity. qNaNs always have one bit set, but there is no requisite bit set for the sNaN.

https://github.com/llvm/llvm-project/pull/102790