xtensa: Minor fix for FP constant synthesis
This patch fixes an non-fatal issue about negative constant values derived
from FP constant synthesis on hosts whose 'long' is wider than 'int32_t'.
And also replaces the dedicated code in FP constant synthesis split
pattern with the appropriate existing function call.
gcc/ChangeLog:
* config/xtensa/xtensa.md:
In FP constant synthesis split pattern, subcontract to
avoid_constant_pool_reference() as in the case of integer,
because it can handle well too. And cast to int32_t before
calling xtensa_constantsynth() in order to ignore upper 32-bit.
gcc/testsuite/ChangeLog:
* gcc.target/xtensa/constsynth_double.c:
Modify in order to catch the issue.