RISC-V: testsuite: xtheadfmemidx: Rename test and add similar Zfa test

Test file xtheadfmemidx-medany.c has been added in b79cd204 as a
test case that provoked an ICE when loading DFmode registers via two
SImode register loads followed by a SI->DF[63:32] move from XTheadFmv.
Since Zfa is affected in the same way as XTheadFmv, even if both
have slightly different instructions, let's add a test for Zfa as well
and give the tests proper names.

Let's also add a test into the test files that counts the SI->DF moves
from XTheadFmv/Zfa.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadfmemidx-medany.c: Move to...
	* gcc.target/riscv/xtheadfmemidx-xtheadfmv-medany.c: ...here.
	* gcc.target/riscv/xtheadfmemidx-zfa-medany.c: New test.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>