fix(input): placeholder hidden when there is also a label on IE11 (#11674)

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## PR Type
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[x] Bugfix
[ ] Enhancement
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## What is the current behavior?
If a `md-input` has both a `placeholder` and a `label`, the placeholder is invisible on IE11.
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Issue Number: 
Fixes #11668

## What is the new behavior?
If a `md-input` has both a `placeholder` and a `label`, the placeholder is visible on IE11.

## Does this PR introduce a breaking change?
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[ ] Yes
[x] No
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## Other information
Verified manually in IE11 on Windows 10. No unit tests added as we don't run them against IE11.
Also verified by the reporter in #11668 (comment).

Author: Splaktar

src/components/input/input.scss

@@ -148,7 +148,7 @@ md-input-container {
     opacity: 0;
   }
 
-  &:not( .md-input-has-value ) input:not( :focus ) {
+  &:not( .md-input-has-value ):not( .md-input-has-placeholder ) input:not( :focus ) {
     color: transparent;
   }
   &:not( .md-input-has-value ) input:not( :focus )::-webkit-datetime-edit-ampm-field,