Implement IEEE immediates for binary32 and binary64.

Clarify the textual encoding of floating point numbers.

Don't allow decimal floating point since conversion to/from binary can produce
rounding problems on some (buggy) systems.

cranelift/docs/langref.rst

@@ -212,6 +212,9 @@ indicate the different kinds of immediate operands on an instruction.
     signed two's complement integer. Instruction encodings may limit the valid
     range.
 
+    In the textual format, :type:`imm64` immediates appear as decimal or
+    hexadecimal literals using the same syntax as C.
+
 .. type:: ieee32
 
     A 32-bit immediate floating point number in the IEEE 754-2008 binary32
@@ -229,6 +232,38 @@ indicate the different kinds of immediate operands on an instruction.
     bits of the operand are interpreted as if the SIMD vector was loaded from
     memory containing the immediate.
 
+The two IEEE floating point immediate types :type:`ieee32` and :type:`ieee64`
+are displayed as hexadecimal floating point literals in the textual IL format.
+Decimal floating point literals are not allowed because some computer systems
+can round differently when converting to binary. The hexadecimal floating point
+format is mostly the same as the one used by C99, but extended to represent all
+NaN bit patterns:
+
+Normal numbers
+    Compatible with C99: ``-0x1.Tpe`` where ``T`` are the trailing
+    significand bits encoded as hexadecimal, and ``e`` is the unbiased exponent
+    as a decimal number. :type:`ieee32` has 23 trailing significand bits. They
+    are padded with an extra LSB to produce 6 hexadecimal digits. This is not
+    necessary for :type:`ieee64` which has 52 trailing significand bits
+    forming 13 hexadecimal digits with no padding.
+
+Subnormal numbers
+    Compatible with C99: ``-0x0.Tpemin`` where ``T`` are the trailing
+    significand bits encoded as hexadecimal, and ``emin`` is the minimum exponent
+    as a decimal number.
+
+Infinities
+    Either ``-Inf`` or ``Inf``.
+
+Quiet NaNs
+    Quiet NaNs have the MSB of the trailing significand set. If the remaining
+    bits of the trailing significand are all zero, the value is displayed as
+    ``-qNaN`` or ``qNaN``. Otherwise, ``-qNaN:0xT`` where ``T`` are the
+    trailing significand bits encoded as hexadecimal.
+
+Signaling NaNs
+    Displayed as ``-sNaN:0xT``.
+
 Control flow
 ============