[NFC][PowerPC] Fix spe.ll to work with update_llc_test_checks.py again

Using split-file does not work with update_llc_test_checks.py. It's also
mostly redundant, as the single and double tests can just use a single
llc and FileCheck invocation for each FPU type using -check-prefixes
rather than -check-prefix, and update_llc_test_checks.py will merge what
it can. Only test_dasmconst needs to be SPE-only and so is pulled out
into its own mall file (rather than using sed to preprocess the file and
keep it commented out for EFPU2, which would work, but is ugly).

As well as cutting down on the number of RUN lines, this also results in
test_fma's CHECK lines being merged for both FPUs.

Reviewed By: kiausch

Differential Revision: https://reviews.llvm.org/D106969