[x86] try to keep FP casted+truncated+extracted vector element out of GPRs

inttofp (trunc (extelt X, 0)) --> inttofp (extelt (bitcast X), 0)

We have pseudo-vectorization of scalar int to FP casts, so this tries to
make that more likely by replacing a truncate with a bitcast. I didn't see
any test diffs starting from 'uitofp', so I left that as a TODO. We can't
only match the shorter trunc+extract pattern because there's an opposing
transform somewhere, so we infinite loop. Waiting to try this during
lowering is another possibility.

A motivating case is shown in PR39975 and included in the test diffs here:
https://bugs.llvm.org/show_bug.cgi?id=39975

Differential Revision: https://reviews.llvm.org/D64710

llvm-svn: 366098